Ryan Carson, founder of Treehouse talks about how it's possible to build a successful startup - alone. Alone, in the sense that you don't need a co-founder.
While I agree more people may hinder decision making or even cause unhappiness over work schedule differences, not having someone else for emotional support at times of 'low' can equally be an issue.
Running a product used by hundreds to thousands of people is not easy. The larger your active user base, the harder it is to manage. And when a few very vocal users starts screaming at you, it's hard to keep the spirits up. They can sometimes be destructive and they can make or break your business. What you need is someone who can give you that emotional support needed to ride it through.
An excerpt from an ex co-founder of Y Combinator:
The low points in a startup are so low that few could bear them alone.
This is especially true and it makes having that 'one other' vital to a more successful journey.