I believe we’re social beings, and that we succeed or fail based on the quality of our interactions with others. Such interactions introduce accountability and are educational and motivating.
Jake Edge reports that when Linus Torvalds was asked what motivated him to write Linux, the answer was that
If Linux hadn’t attracted other people right away, the project probably would have died within six months. But the involvement of others was a “huge motivating factor” for [Linus] to continue, because it “made it much more fun“.