Being myself a summer applicant sans co-founder, it's important to differentiate between having a co-founder and wanting a co-founder. If you didn't want a co-founder, that would probably be an issue (it's way too risky to put the entire load of a start-up on one person). But not being able to find a suitable co-founder is another story. It's not something to rush into just so you can say you have one.
