Disclaimer: I don't own Harry Potter.
He wants to cry because everyone's lost someone important to them. He wants to cry because even though the war's over, people still have reason to mourn.
He wants to cry as he watches Dennis Creevy sit on the cold floor beside the body of his brother, fingering the camera (it's all they've got left of Colin). The look in Dennis's eyes isn't one people expect to see in the eyes of someone so little. It almost completely overrides everything youthful, bright and childish (except vulnerability—because he's got no one to sit there with him at his brother's side as silent sobs wrack his tiny body).
He wants to cry as he watches Andromeda, with baby Teddy folded tight to her bosom, clutch Tonks' pale, cold hand tight, intertwining her fingers with her only daughters (those fingers who'll never clutch hers back). Andromeda's sobs are much louder than Dennis', but her tears are fewer. She has Teddy, and she won't let anyone rip him away. (And that in itself is something to move on for.)
He wants to cry as he watches the Weasleys all crowd around Fred's body, multiple different shades of bright red pressed close together to fill the empty gap right in the middle—where Fred always stood. He pretends not to remember the boggart in the drawing room at Grimmauld Place, of a dead Fred (but he doesn't need to, the real thing's lying there, and that's even worse). Their pain is one he can't expect to understand fully, so he stands aside. He isn't welcomed in their suffering.
He wants to cry as he watches the Patil twins, hugging and crying, no longer completely identical. He can't miss the long, jagged wound from Padma's left eyebrow down to her chin, and the burn marring Parvati's chin and lower lip. They're no longer identical in appearance, but their pain knows no differences.
He wants to cry as he watches Narcissa Malfoy cradle her son tight to her side, their sadness a different kind, but heart-breaking nonetheless. He feels pity—and he hates himself for it; there are people who deserve it more, he believes. But they've lost parts of themselves, and the faded scars on their forearms are painful reminders of what happened to all those years. And their faces blur until all he can really see is a mother who loves the child she can't remember, and the son who seeks comfort from the mother he never did really know.
He wants to cry as he watches Neville and Luna sit together and cry, their sadness is clipped, shorter and more incomplete, but the sharp edges are more painful. They haven't got anyone else to sit with and mourn; no one close enough knows their pain. They've both lost enough. Classmates, teachers, friends … people they expected to see live on, are dead. And Luna's father, and Neville's grandmother, wouldn't understand. So they sit to together and weep, for the people all around them. But their cries are hollow, and not as meaningful. They have each other. This probably won't change.
He wants to cry as the sobs, sniffles, wails and cries echo through the school, tainting the peaceful silence which marks the end of the war. And maybe, he discovers, the end is only the beginning.
And suddenly he doesn't want to cry, because he knows that life goes on for those who are alive, and so does love. He smiles faintly as Ginny's slightly scarred hand slides into his own.
Even though the war is over, he won't stop fighting.