I have that same problem! I wonder if it's some sort of defense mechanism, mine usually happens when I'm nervous and it's too quiet. I did laugh at a funeral, and it's not that I thought anything was funny, I was sad, it just got quiet, and it seems to happen during quiet times like that, or when it's not supposed to! Same thing back when I was in school, if somebody was giving a speech and it got quiet, it'd happen then, too. I actually worried in advance about the "laughing in a funeral or during class when it got quiet" thing. I think it might be nerves with me. The good thing is, as I've gotten older, it happens much, much less. I hope the same thing happens for you, too!