I've been reading this article ("What’s So Friggin' Funny?" [Link]). The interesting point, to me, is that laughter has not much to do with a really funny punch line of a good joke, but is basically a form a socializing and it facilitates interacting with others.
I do laugh occasionally. I tried to observe myself both when I'm alone and when I'm interacting with others and found it quite curious that I laugh more than I thought I would (which is still not that much). It certainly depends on the mood I am in; I'm usually quite apathetic, but still experience some mood swings every now and then. But what I found most striking was that I actually do laugh when I'm alone. When I read something funny or think about something I find amusing, I actually laugh in pretty much the same way I do when I'm not alone, so it is certainly not only a way of interacting for me.
How about you?