I don't think motivation only comes from relationships, though I don't feel so much motivation when I'm on my own. My life seems to go round in ever-decreasing circles when I'm alone for too long. So I agree, a lot of motivation is social. I don't like to let people down, I don't like to seem the only one in a group who doesn't do anything. Even answering questions like this on WP, if it weren't for the questions coming in from out there, I doubt that I'd think of the topics myself and write down what I thought. Commitments, even awkward ones, seem to keep me alive somehow. If nobody cared what I did, then I wouldn't do much.
People help me to think of things. I have great trouble picking one thing from a list, and they often save me the bother by doing it for me.
Also they have ideas that sometimes interest me strongly, ideas that I'd probably not stumble on if I were alone. Somebody the other day was talking about the placebo effect and its implications for the ability of the mind to control fear, pessimism, low spirits, etc. I'd been feeling rather sluggish and negative for weeks, but after hearing the stuff about placebos, I magically started to feel better as I questioned whether my negative feelings were really as out of my control as I'd been assuming they were.