I'm sure it's two-fold. YC filters out people who aren't committed. Also, having someone believe in you makes you believe in yourself--it becomes a self-fulfill

I'm sure it's two-fold. YC filters out people who aren't committed. Also, having someone believe in you makes you believe in yourself--it becomes a self-fulfilling prophecy. This works in a negative sense, as well. For example, when pg says your chances of succeeding without a cofounder are low, if you internalize that, you won't succeed without a co-founder, because "why try" if you can't succeed?