Laughing by yourself?

[ItalianFX]

One of my roommates is down in the living room watching tv and he is laughing by himself. I find this really lame and weird. He does it all the time, even when he is in his room watching movies.

If I am sitting here watching tv, or watching a video, and I find it funny, I might laugh to myself, but he is making a full out laugh like he can't hold it in.

Does anyone else laugh by yourself when no one is around? And what is your opinion on the matter.