(Character centric.
Disclaimer: I don't own Reborn!)
Tsuna smiles because that's all he can do, because sometimes it's like that's all he's capable of. He smiles because he has to let everyone know he's fine and that everything is okay. Tsuna smiles because he needs to reassure himself.
Gokudera smiles because he's thinking of winning, of Tenth, of the life ahead. He smiles because he's going to get a good grade or because they're going to come out alive in this crisis and he smiles for tenth because then tenth smiles. Gokudera smiles because he's thinking of the future.
Yamamoto smiles because he's having fun. He smiles because this is all just a game and he's clueless to the truth behind it. He smiles because everyone else is and they're obviously having fun and he smiles to join in. Yamamoto smiles because this is all a game but some are taking it too seriously.
Hibari smiles because he's found a new prey. He's found something amusing and entertaining to poke and beat. He's found a new toy to break or try to. He smiles because he's winning and because he's slowly and quickly taking apart something. Hibari smiles because he's a predator.
Mukuro smiles because no one knows what he's thinking. He smiles because it makes him mysterious and it makes him hard to read. He smiles because no one can see through it and it makes him dangerous. He smiles because everyone else gets unnerved. Mukuro smiles for his own benefit.
Xanxus smiles because he's amused. He smiles because this is so damn funny whatever it is but it's not really funny to everyone else. He smiles because people cower and that's funny to him because he's the Tenth Vongola. Xanxus smiles because this is all a drama.
Squalo smiles because he's confident. He's confident in what he does and what he'll do and even what he's done. He has no regrets because regrets aren't a sign of pride and that's what Squalo is. Squalo smiles because he's arrogant with reason and because he'll probably win this and that and everything else. Squalo smiles because he's a winner.
Belphegor smiles because he loves to watch people squirm. He loves to watch them look away as he smiles that over bearing smile and stares at them without them really knowing. He loves to watch them get paranoid because he's smiling like he's actually thinking about doing something. He smiles because he's not completely sane but enough so to enjoy unnerving others. Belphegor smiles because he likes to make people crazy.