I think that one of the reasons you might laugh might be the same reason others cry at a happy situation. IMHO it is just a way to express an emotion. The problem is everyone has a preconceived notion of what is an acceptable expression of emotions for certain situation.

When I am in a situation where someone expresses the opposite emotion from the norm I try to remind myself that they might have a different concept of what is acceptable or are just releasing their emotions in the only way that they can at that moment.