That's probably got a lot to do with how Cloud changed toward the end of the game, and what likely happened to him in the aftermath. He got way more sombre and self-doubting after his experiences in the Lifestream, when he realised that most of his adult memories were delusion. He blamed himself for an awful lot that had happened, and once it was all over I can easily see him sliding into a funk of regret and purposelessness.

Considering everything he went through, and everything the world went through, I'd have found Advent Children a lot harder to swallow if Cloud had been a happy little wellspring of joy.