I can think of three reasons to not have a co-founder that are not necessarily relevant to the strength of the venture: (1) The venture is not in an area that'

I can think of three reasons to not have a co-founder that are not necessarily relevant to the strength of the venture: (1) The venture is not in an area that's trendy right now, making it something where people who don't share the founder's belief in the idea and vision might be much more reluctant to take the plunge. But that doesn't mean it's a bad vision. (2) The founder has had trouble finding a co-founder he/she can trust, who is compatible, and who is available for a venture. In a startup scene full of hucksters and flakes, trust is a legitimate concern. (3) (Not applicable to YC since YC is bay area only, but...) The founder may be in a place with a much smaller or non-existent startup scene, like a small town or a city/country where this stuff is rarer. In that case co-founders might be pretty hard to come by.