I've done it both ways (previously with a co-founder, now on my own). They're totally different businesses, though - I couldn't have done the last one without a co-founder.
My previous company was in the online retail space, and now I'm doing SaaS. The process of getting a physical product from a manufacturer to a customer is just much more involved. It took a minimum of three people just to operate the business. I think a co-founder is necessary when you lack the domain knowledge to operate and grow the business yourself, or the business itself requires more hands on deck and you're unable to pay an appropriate wage.