I don't know about others but for me it's very hard to motivate myself to workout at home in my living room, even if I consider myself self motivated. Even at a gym by myself is better than at home. I'm mostly by myself at the gym anyway since I train so late.
Haha, I'm exactly the same. I can't ever bring myself to work out at home. I just can't do it. I don't know why.
I've got a 24-hour gym nearby, though, so I'm always there at weird hours of the night, often entirely alone.
I think that there's something about the gym -- the atmosphere and the way it smells of rubber and iron -- that can't be replicated at home. Also, there are far fewer distractions. Lastly, I like working out with machines alongside freeweights, and I could never collect a large number of machines at home.