IMHO, I think it's a reflexive reaction; I don't have to think about laughing, I just do it. If I think about laughing, I can't laugh; I can't make myself laugh deliberately. Sometimes I laugh when I think about funny things, but I have to feel the humor of the situation. Sometimes I laugh when I'm nervous, or I try to make other people laugh when I'm nervous. It takes the edge off the tension in the situation for me.