Silly Snape Headcanon:
He doesn’t *really* laugh much because when he does, his voice goes up several octaves as he laughs. It’s great, but he’s really self-conscious about it.
On the subject of Snape teaching Harry, it often strikes me that JK always goes for the "Snape is a petty vindictive man" which, IMO, often makes me think that either she doesn't believe Snape to be a particularly bright person, or particularly complex. The moment the discussion went to question Snape's disposition a part of me said "JK would probably just say he was blinded by hatred, like always", which doesn't really paint a good light on his intelligence. (1)