Spoilers for Angel through season 3, episode 9, "Lullaby."
Title is from T. S. Eliot's "Journey of the Magi."
Nothing that isn't mine, is mine.
If he were the sort to wonder about that sort of thing, he would wonder why nothing else qualifies as a moment of perfect happiness. (Wesley wonders, sometimes, but he isn't the sort to ask.)
The axe twirls in his hand, hissing as it slices through the air. "Behind you!" - that's Gunn - and he ducks, dropping to one knee and one hand, and flings his weapon. It flies in a perfect arc, still spinning, and he has gauged the momentum perfectly because it flies into the demon's chest with a satisfying thunk, and the world is saved again...
"You're safe now," he tells her, "safe with me," and she nods. Tears still quiver in her eyes but a smile creeps onto her face. He's telling the truth for once, she's safe, everyone's safe, no one's dead or even hurt, and he's offset his heavy balance of guilt, if only by a single life...
"I love you," Buffy whispers, and in that moment it's all he wants, it's all he ever wanted...
...and the world doesn't end. He's as happy as he knows how to be, but his soul stays with him, he doesn't go evil and slaughter everyone he loves.
The truth is, happiness isn't enough. The pain is always there, tucked away in the back of his mind, the pain and the shame and the guilt. A voice whispering the world would be better off without you, murmuring filth, a blot on the face of the earth, screaming in his ears over and over you deserve to die. The weight of infinite guilt. He doesn't complain, it's only what he deserves, but the voice (his curse, his cross, his conscience) is always there, tempering all his happiness just enough that it's never truly perfect.
What he needs is much simpler, really: distraction.
Lots of things are distracting, but, the way Angel lives, they're mostly not happy. Walking through the agonizing hall of crosses, plunging his hand into the holy water that cleanses and burns, that's distracting enough to take his mind off the old shame, but it's not happy.
And then Darla appears in the door of the hotel, her stomach grotesque and swollen and impossible. Vampires can't have children.
Time inverts itself. Instead of the infinite counting up of endless years (two hundred and forty one, two hundred and forty two, two hundred and forty three) he's suddenly counting down day by day.
There will be no birth, only death. He thinks he should be afraid, that the terrifying happiness growing in his heart should shrink and die, but birth and death are confused and intertwined for him anyway. It's the whole shanshu thing, or maybe it's the vampire thing, born through dying.
(Actually, it's not that at all. He's simply and absolutely certain that the baby won't die, because the baby can't die, because this is his baby and it is not going to die. True a priori, case closed. He doesn't put that into words, though, because the primal certainty is more reassuring, more true, than any words could be.)
When he picks up the tiny, mewling baby from the alley (all grime and cigarette butts, and his baby is in it, which is not all right), it's not the happiness he expected. It's the weight of the world, which he's used to carrying, granted, but he's never felt anything like this before, this life, tiny and helpless and human, utterly dependent on him. And it's crying.
It's not until later that he realizes what else changed as he picked up his son. As he bent to pick up the baby, he set down everything else, and suddenly he doesn't care about all the people he slaughtered, would kill them all over again and twice as painfully if Connor needed him to.
Babies, as it turns out, are very distracting.
Connor cries a lot. Angel reads every parenting book Fred can find him, turning pages with one hand while he rocks his son with the other, and comes to the conclusion that he believes in attachment parenting. He picks up the baby when he cries, feeds him when he's hungry, and his world is filled with Connor: the curve of tiny fingers, the sounds of different cries (high-pitched keening for hunger, soft and insistent for a wet diaper) and the smell of talcum powder. This is what's important. The world can go to hell.
And then one day, after his feeding and before his nap, Connor smiles back up at Angel, and it does.
It's three weeks and seven thousand dead before Wesley puts a crossbow bolt through his heart.