On Remus Lupin's first birthday, he got loads of toys and gifts and attention from his family. He doesn't remember it, but he wonders what could have been going through his tiny brain. He likes to think that he felt loved and appreciated in some way.
On Remus Lupin's fifth birthday, he was holed up in a shed in the backyard, his small werewolf body shaking with the effort not to rip himself apart. It didn't work, but when he transformed back he didn't understand why he had so many scratches and marks. He also didn't understand how he had missed his birthday.
On Remus Lupin's twelfth birthday, he was given the best gift he could have thought possible – the promise of three wonderful boys to be his friends, forever. Sirius decided they should call themselves the Marauders and Remus was hard-pressed to find a time, place, or group of people with which he felt he belonged more.
On Remus Lupin's sixteenth birthday, his friends did something he never would have thought possible – something so entirely generous and selfless that he felt in awe of them. They became animagi. For him. He didn't know how to react, but he discovered during the next full moon that his transformations were not only bearable with them – they were fun. They were exciting and full of adventure and Remus could find no true way to thank his friends.
On Remus Lupin's seventeenth birthday, he got the gift he had wanted for the past three years. The boy with dark hair and long limbs crawled into his bed and told him that he wanted him and Remus was sure his heart would burst from joy. He kept asking why as Sirius slid off his shirt and the only response he received was a mouth on his own, warm and gentle and utterly perfect, and hands sliding down his body, ridding him of the chill that went so deep.
On Remus Lupin's twenty-second birthday, he spent the night alone in his flat, desperate for the company of his three friends – two dead, one traitorous. He couldn't think of anything but how much he wanted his life back.
On Remus Lupin's thirty-fourth birthday, he realized he had the true respect of James' son, and nothing pleased him more. His emotions were rattled because of Sirius' escape, and he was unsure if he would have the nerve to turn Sirius in, even if he was able to catch him.
On Remus Lupin's thirty-fifth birthday, he drank good wine and made love to Sirius twice, his heart slowly healing from the hurt it had endured over the years.
On this, the day of Remus Lupin's thirty-seventh birthday, he realizes that is alone, once and for all. There is no going back, and no one to love him and comfort him and care for him the way only Sirius could. He is hardened by years of pain and mistrust, by betrayal and the desperation for good that never comes. He is broken.