Love was both his Boggart and what he'd witnessed the one time he'd glimpsed into the Mirror of Erised. It was why he'd built walls around his soul, mastered Occlumency, lived in a dungeon during most of the year. He wanted to keep love away forever. It was foolish and risky and not worth paying the price of the heartbreak that was surely to come his way. Love was the warm red and gold of Gryffindor, not the chill of Slytherin's green and silver; the affectionate, comforting smile of Remus Lupin, not the sarcastic, disdainful sneer of Severus Snape.
It shocked him to see the same image in the Mirror as the one that had shown itself when he'd found a Boggart in his old linen closet. He supposed it couldn't be helped, and guarded himself against it.

Amazing, how some things slip through the strongest barriers.

Who'd have known his life would include it? He'd spent most of his young days alone – alone in a corner, hidden from his abusive father. Alone, wandering the corridors of Hogwarts, alone even with the chatter of his "friends" around him. Alone when he joined the Dark Lord, alone and pathetic when he realized his mistake and went to recant to Dumbledore.
His life was a series of nightmares. Nearly killed by his father, nearly killed by himself, spending hours staring at the small glass vials that kept the lethal poisons he'd brewed illegally, in secret, in his sixth year. Nearly killed by Lupin that night, when he'd been foolish to listen to that idiot Black. The face of the werewolf soon became his most terrifying nightmare, snarling and slavering, and he couldn't keep the venom out of his voice when talking to Lupin, especially when the boy apologized continuously, claiming he'd never planned the act along with his friend. But Severus was not one to let go of grudges. His sharp tongue protected him from being associated with the werewolf during the rest of his school days.

Who'd have foreseen the romance that had somehow crept its way into his life? Slinking into his mind years later, with the bars of Honeyduke's best, silently appearing on his desk every Thursday during his free period in the afternoon. They came without a note, or any trace of why, how, or who…
Soon enough, he became aware that that the friendly glances were no longer as quick and subtle as they had been, but became more intent, and the quiet, well-mannered werewolf was often caught gazing in Severus' direction, to be brought out of his reverie by the ever-twinkling Headmaster, who subtly seemed to notice everything.
Severus noticed that he would be fooled by the trick steps in the staircases that he'd known to jump for years, and the walls that pretended to be doors seemed to snigger at him as he tried to pry them open. He noticed he was no longer blasting apart the rosebushes on Valentine's Day, and the sparkling rubies that rested in Gryffindor's hourglass were no longer taking the abuse he'd shown them so many times before. It couldn't – no, it couldn't have anything to do with the fact that it was the same house that Remus Lupin had lost and won points for in his own childhood. He couldn't explain the sudden warmth that stole its way into his heart when Lupin's eyes settled upon him.
And when he stared at his own reflection, he couldn't see what someone else would be attracted to. But for once, he didn't hate himself.

To Slytherins, never being loved didn't necessarily mean never having a family, though for Severus, the two would have to go hand in hand. He didn't at all like the notion of marrying a relative, however distant. He'd never preferred women anyway.

Love came along with the chocolate bars, even when they were missed once a month at the full moon; he received two the next time.

A relationship was slowly created, with hesitant touches on the shoulder, on the arm, comfortably sitting next to Remus at Headquarters.
Once Severus had collapsed after a particularly dreadful meeting with the Dark Lord, and Severus didn't notice he was being carried to a bed until he was halfway up the stairs, wordlessly watching as Lupin made him comfortable, falling asleep, soothed by the warmth of the werewolf watching over him.
The next morning he woke with a cry as he realized his barriers had been broken, and all the words he'd always needed to say streamed out of his mouth, and he found himself confessing and apologizing, and nearly revealing what had happened to his battered heart.

Heartbreak came only in the form of the miserable, slumped shadow of Remus mourning Black. Severus knew he would break because of it.

He let it. It wasn't as if he could stop anything anymore.