What you do when something is
funny. Of course, some of us also do it when something is
sad or
horrible or just plain
disturbing. I don't know why, it seems to be an instant
unconscious reaction that leaks through at
inappropriate moments. I never know whether to let it burst out or hold it inside me because it is
rude.
However, sometimes laughing inappropriately can be fun. In theatres, in quiet classrooms, I often think of a joke I heard the day before and chuckle with abondon. It may cause people to stare but at least it is real, as opposed to a pity laugh.
Note also that laughing is the older and more mature sibling of giggling.