Please note, this was set in the time when Harry was in his 5th year, and the memories of Sirius are set in the time of the marauders - Thanks!

12 Grimmauld Place. Sirius, Harry, 6 of the Weasleys, Lupin and Tonks were all sitting around the large, wooden table in the kitchen. They had all been talking for hours, but now the conversation had moved to the topic of 'love'...
"So Sirius... How come you never married? Have you never been in love..?" Harry taunted, having a joke with his god-father.
"Haha, not for me no! I'm not the loving person!" Sirius joked, but as he looked away, Lupin caught his eye, a sympathetic look upon his face.
You see, Sirius lied to Harry. He had been in love before... Sirius let the others talk as he leant back in his chair and started thinking of when he was in his 5th year at Hogwarts...

It was a cold and frosty January morning and Sirius hurried to charms class with James. They were already 5 minutes late. They sneaked into the back of the classroom smug looks on their faces. It was their 2nd period of the day, and the Gryffindors had charms with the Ravenclaws. Siruis was just casually leaning back in his chair and talking to James when Professor Flitwick ushered the class into silence.
"Today, we're practising the banishing charms I was talking to you about yesterday! We will be practicing on these cushions here! Turn to page 445 in your textbook for some handy tips!" He squeaked. "I will also be putting you into pairs!" He added.
Flitwick hopped around the class, pairing people up.
"You're not with James today Sirius I'm afraid! You will be working with Lily, James... And you Sirius, will be working with Ruby" Flitwick said, gesturing for a girl to come over.
Sirius looked in the direction where Flitwick was gesturing and stopped in his tracks. He saw the prettiest girl, he had ever seen. She had red-brown hair with warm brown eyes, long eyelashes and a slender shape. She was from Ravenclaw.
"Erm, hi, I'm Ruby... And you're Sirius" She said in a shy way, offering her hand.
Sirius shook it and just stared at her.
"It's okay, I've done banishing charms before, they're quite simple" She said confidently. And in under 10 minutes, she had banished all 12 of the cushions they were meant to be working on.
"There. Now that's the work out of the way, lets get to know eachover!"
So Ruby and Sirius spent the rest of the lesson talking about themselves to eachover, getting to know eachover, talking about their families and their favourite things. Ruby found Sirius very funny, and her stomach really hurt by the end of the lesson...
"This lesson, was good. Do you want to meet up outside of lesson? Like, hang out at the weekend or something?" Sirius said, looking at Ruby hopefully.
"I'd love that! Well, we can always use the Hogsmede trip at the weekend, if you wanted?" Ruby suggested, grinning.
"Sure, meet me by the great hall on saturday then!" Sirius said, also grinning now. "And don't be late" He called over his sholder as he walked off.

That night in the common room, Sirius told Remus, James and Peter about his date at the weekend. It's all he talked about. James fell asleep in the common room as Sirius was too excited to go to bed yet, and Remus just said:
"Sirius, just, stop, for a minute, please? How do you know you even, like her yet? You see, you only just spent one lesson together, how do you know that she's not a horrible person?"
Sirius looked utterly offended and replied with "She gives me this funny feeling, like, I've got birds in my stomach or something" He had a funny look on his face.
The apparent asleep James snorted and opened his eyes. He started to laugh and couldn't stop.
"Oh shut up and pine over Lily or something." Sirius snapped. That shut James up.