I think it depends on the type of show I'm watching. If it's like an action type of gory type of thing, I don't mind watching it on my own, only because most of my friends don't really like that type of stuff anyways. But if it's like trash TV, I have to watch it with a friend because it just makes it funnier.
If it's sports, I like watching TV with other people. If it's like a scripted show, I want to be immersed as much as possible, so I'll watch that by myself. Unless it's something that my wife and I watch, and then we'll watch it together.