From Pottermore:
"This time, the group included an Auror who had been too young to belong to the Order during its first incarnation. Clever, brave and funny, pink-haired Nymphadora Tonks was a protégée of Alastor 'Mad-Eye' Moody, the toughest and most grizzled Auror of them all.
Remus, so often melancholy and lonely, was first amused, then impressed, then seriously smitten by the young witch. He had never fallen in love before. If it had happened in peacetime, Remus would have simply taken himself off to a new place and a new job, so that he did not have to endure the pain of watching Tonks fall in love with a handsome, young wizard in the Auror office, which was what he expected to happen. However, this was war; they were both needed in the Order of the Phoenix, and nobody knew what the next day would bring. Remus felt justified in remaining exactly where he was, keeping his feelings to himself but secretly rejoicing every time somebody paired him with Tonks on some overnight mission.
It had never occurred to Remus that Tonks could return his feelings because he had become so used to considering himself unclean and unworthy. One night when they lay in hiding outside a known Death Eater's house, after a year of increasingly warm friendship, Tonks made an idle remark about one of their fellow Order members ('He's still handsome, isn't he, even after Azkaban?'). Before he could stop himself, Remus had replied bitterly that he supposed she had fallen for his old friend ('He always got the women.'). At this, Tonks became suddenly angry. 'You'd know perfectly well who I've fallen for, if you weren't too busy feeling sorry for yourself to notice.'
Remus's immediate response was a happiness he had never experienced in his life, but this was extinguished almost at once by a sense of crushing duty. He had always known that he could not marry and run the risk of passing on his painful, shameful condition. He therefore pretended not to understand Tonks, which did not fool her at all."
Remus wasn't sure about a lot of things, but one thing he did know was this: he needed a drink. It isn't often that one actually needs a drink. It's the sort of thing that is said often – bandied about in a cavalier sort of way, after someone's had a long day at work, for example – but rarely truly justified. Often people want a drink, they fancy one – but they don't need one, not in the way that Remus Lupin needed one now. He had too many thoughts in his head, and many of those thoughts were just too painful to consider. They gnawed into his stringy sinews and twisted red hot into his guts. He needed something – anything – to get rid of those thoughts. Something to bury them deep underground, to help to lower them into a cavernous grave and shovel soil roughly on top.
It was nearly midnight, so The Leaky Cauldron would be shut. Not that Remus minded – he didn't want to be somewhere where there would be too many people, much less somewhere he might see he knew. No, he needed somewhere far from any happy people, a little darker, and with a less stringent attitude towards licensing hours. It was with this in mind that he disapparated into the night with a sharp crack.
Cold air washed over him; he gratefully drew in a great lungful as he appeared into an empty, narrow street. Pulling his cloak around him, he strode purposefully into the inky darkness. At the end of the street, a shabby wooden door nestled into surrounding brickwork. No sign over the entrance, but Remus knew what was inside – a tavern which specialised in strong drinks and a lack of conversation. It was the sort of establishment that attracted unsavoury sorts: goblins, vampires, dark wizards – and werewolves.
The door to Grimmauld Place creaked open several hours later, the tired moan from the hinge wafting into the dimly lit hallway. Remus placed one hand on the wall to steady himself as he eased the door shut, shushing it gently as he did so. Carefully, he began to move towards the main staircase. Every step was an effort, his feet heavy on the dusty carpet. He edged closer. Almost there, almost in, not far to his room…
A sudden clatter as he caught the coat stand with a trailing arm. Remus froze. A heartbeat and then –
"HOW DARE YOU WAKE ME AT THIS TIME OF THE NIGHT?! FILTHY WEREWOLF, HAVE YOU NO RESPECT? BESMIRCHING THE NOBLE HOUSE OF BLACK, YOU –"
The curtains over the portrait of Mrs Black whipped open. Somewhere above him, Remus heard a moan of frustration and the thump of someone jumping out of bed. A door slammed open and a tall man with dishevelled dark hair came bounding down the stairs.
"Bloody hell, what time is it?! Shut up, bloody hell, shut up!" Sirius Black jabbed at the portrait with his wand, bringing the curtains shut. The screeching stopped as abruptly as it had started. Sirius turned to face Remus, rubbing his eyes petulantly. "Moony, what the fuck are you playing at? It's 4am! You're bloody lucky it's just me here tonight. Why are you back so late?"
Remus blinked, hard, trying to focus on the face which swam in front of him. He stumbled slightly as he stepped forward, his hands raised in a vague apology.
"Padfoot, I'm – I didn't – sorry", he mumbled.
"Moony, are you drunk?" Sirius looked incredulous. "You are, bloody hell!" He suddenly grinned, grabbing Remus by the arm and steering him towards the kitchen.
"Pads, I'm don't –" Remus's incoherent reply was lost as he was bundled into a chair.
"Moony, you're absolutely fucked. This is fantastic, I haven't seen you drunk since we were about nineteen. And even then, I wasn't sober enough to remember it!" Sirius smirked, apparently now unbothered by the time. "Who'd have thought that I'd live to see the day that Remus Lupin, prefect of Gryffindor House and all-round goody-two-shoes, tried to sneak in unnoticed after a skin full".
Remus shut his eyes and lowered his forehead on to the table. It was cold and smooth.
"All right then, what happened? I thought you were on Order business tonight?"
Remus did not answer. Of all the people in the world, Sirius was quite possibly the one he wanted to talk to least at this point.
"Moony!" Sirius jabbed him in the shoulder. "Moony, come on. I mean, this is hilarious, but why on earth are you so smashed?"
"I – I just went to have… to have a drink after I was finished with – well you know, the Order stuff."
"Yes, yes, I know you can't talk about what you were doing. Super-secret and all that." There was only a touch of bitterness in Sirius' voice at this. "And sorry, 'a drink'? More like about ten, by the sorry state of you. Who were you with?"
"On my own."
"On your own? Since when do you go out on your own and get so drunk?" The merriment faded from Sirius's voice as he placed a hand on Remus's shoulder. "Was everything OK? With the Order? Is everyone all right?".
Remus snorted. The idea that there was anything that really mattered wrong, anything that meant anything to anyone other than him, seemed strangely funny. Sirius's nails suddenly dug into his arm.
"Moony, what? What's going on?"
"No, no, no, everyone's OK. The Order's OK, everything was great". Remus spoke with the infuriating sluggishness that drunks so often have; Sirius continued to grip him tightly. "I – ow, ow, Padfoot, that's sore!" Sirius let go. "It's all fine, I just wanted to have a drink. On my own".
"Right. Fine." Sirius sat back in the kitchen chair and eyed Remus suspiciously. "That's a bit – well, it's a bit odd, mate."
"No it's not. People do… that".
"People don't normally go out and get absolutely rat-arsed on their own, not unless something's wrong. At least, the Remus Lupin I know wouldn't." There was something about Sirius using his full name that made Remus sit up. Even now, when they were alone they still always used the nicknames they'd coined when they were teenagers – there was something nostalgic, almost soothing about it.
"I don't want to talk about it, Sirius." Sirius didn't reply. Still frowning slightly, he reached over to the cabinet and picked up a goblet. A casual flick of his wand filled it with water.
"Right, well, have some water anyway. You never could handle your booze, you're going to feel absolutely horrendous in the morning". He pushed the water toward Remus, who ignored it.
Some time passed, Remus slouching in the chair and Sirius quietly watching him. The grandfather clock in the corner ticked somewhat ominously; the grand old house was otherwise silent. The pale yellow light from the lamp in the corner seemed feeble in comparison to the moonlight which streamed onto the table between them through a window above. Five days until the full moon.
"Moony, you were a stubborn little prick when we were at school too. Why don't you just tell me what's wrong?"
Remus looked up into Sirius's face, able to focus better now than he had when he had stumbled in. Sirius's cheekbones jutted out, glowing pale in the half-light; his eyes were full, dark and glinting. He's still handsome, isn't he, even after Azkaban? The words echoed around Remus's head.
"She thinks you're handsome, you know."
"Who?"
"She said it tonight."
"Moony, a lot of women have said I'm handsome," Sirius smirked slightly, the ghost of a roguish smile flitting past, before continuing. "Who said it tonight?" Remus shut his eyes tightly. He just felt woozy now, no longer numb.
"Nymphodora Tonks".
"Um, right. OK. I – Moony, you've lost me. Is this why you're upset?"
"I'm not upset."
"Right, right. You just came home at 4am, blind drunk, for no reason."
There was a clatter as Remus stood up suddenly, kicking his chair backwards. His wand was drawn, shakily pointed across the table at Sirius.
"Shut-up, Padfoot! Stop taking the piss out of me! You have no idea how easy it is for you!". Sirius leapt up too, hands held aloft.
"Woah, woah. Jesus Moony, what's wrong with you?! Put your wand away."
"You just don't get it. You always just got what you want, nothing was hard for you. You never understood what it was like to be an outsider, to be such a, a – a monster. What it is like." At this, Sirius snarled, his hand suddenly reaching for his own wand.
"OK Moony, fine. I mean, I was in Azkaban for thirteen years and everyone thought that I was a fucking murderer, but yeah apart from that it's been a piece of cake!"
"That's not what I'm talking about!"
"Then for the love of god, Moony, explain what it is that you're talking about! Because I really haven't got a clue!"
Both men were stood facing each other, wands aloft – Sirius, in his crumpled pyjamas and Remus in shabby robes. Shadows pooled around their feet, their faces half-hidden in the dim light of the kitchen. For a moment, neither spoke. Then, with a sigh, Remus slumped back down in his chair, dropping his wand on to the cold floor.
"I'm sorry, Padfoot. I'm sorry. I'm sorry for everything."
"What are you sorry about?"
"I'm sorry for being who I am. I'm sorry for loving her, when I know I shouldn't". Sirius cautiously took a step towards Remus, and placed a hand on his shoulder again.
"For loving who?" Remus stared at the ground. "Is this about Tonks?" Sirius suddenly pumped a fist in the air, a broad grin spreading across his features.
"I knew it! Yes! I knew you liked her!" Remus looked across at him, his mouth slightly open.
"How did – how did you know?"
"Moony, I've known you since we were 11. And I endured you that entire summer when you were in love with Mary Moriarty in fourth year. I'm not, contrary to what you seem to believe, an idiot." Remus returned his forehead to the table, suddenly feeling both exhausted and rather resigned. "So, what are we going to do about it?" Remus winced slightly at Sirius's use of 'we', but did not look up.
"We aren't going to do anything at all about it."
"Well she probably won't start going out with you if we don't do something."
"It doesn't matter! I can't be in love with her, because she can never be with me."
"Why not? I know you've got a terrible haircut, but I'm sure she can see past that."
"Because of what I am, Padfoot! I can't be with someone, it's too dangerous! It's – it's not right." Sirius rolled his eyes slightly, but stopped when he noticed the expression on Remus's face.
"Moony, I really don't have time for your 'oh woe is me, I'm a werewolf' routine right now. No, be quiet!" He held a hand up sternly, halting Remus's interruption. "Look mate, I know it's shit. And I'd do anything for you to not have your, um, furry little problem." Remus gave a half-smile, in spite of himself, at this. "But the point is, we can't change it, can we? And now more than at any other time, you've just got to get on with it!"
"I can't, Padfoot."
"Look, I know it's hard. But I'm just not going to sit by and let you mess this one up, not again! You deserve to be happy, Moony, just as much as anyone else." Before Remus could respond, Sirius had pulled him into a deep hug. Remus rested his cheek against his old friend's shoulder, and felt a wave of warmth roll over him. For a moment, he was 12 years old again. He was in their dormitory, tears rolling down his face, being held in the same embrace. He was suddenly not alone anymore, the great burden of his secret lifted when his friends had found out. They broke apart and Remus was back, older and drunker, in the kitchen of Grimmauld Place. Sirius gave him a tired smile.
"Now, we aren't going to get this sorted tonight. What you need now, is bed."
"But I – "
"No buts. Speaking as someone with much experience of alcohol, and indeed of hangovers, we need to get you to sleep." Sirius began to gently steer Remus towards the door.
"Padfoot, promise you won't tell anyone about this?"
"Moony, would I ever?"
Sirius smirked and extinguished the lights with a flick of his wand. In the silver half-light, he guided his friend towards the stairs. Tomorrow, there would be time for sore heads, long discussions and restorative bacon sandwiches.