From http://foundrs.com/find-a-cofounder :

(the wrong reasons to want a co-founder:) Yes, it's true that startups with only one founder are more likely to fail. But it's due to signaling: if you, as passionate about your idea as you are, can't even convince one more person to join you, how will you get users excited to download your app, get investors excited about the market, and get Microsoft and Coca-Cola to partner with you?

Solo founders usually fail because they are bad at convincing anyone of anything.