I do watch movies alone for the following reasons :
1. No one else (the gf, the sibs, etc) is interested to watch the movie.
2. The gf is unavailable (due to schedule constraints) and the movie is about to end its run in the cinemas.
3. There is a random urge to watch a particular movie (I'd call this "impulse watching")
I kinda pity some people when they say that they like to watch a particular movie but couldn't find someone to watch it with...what the heck, go watch it alone.