Past the demotivation, PG lists "single founder" as [one of]the top mistake that kills a startup.

"it's a vote of no confidence. It probably means the founder couldn't talk any of his friends into starting the company with him. that's pretty alarming"[1]

"Ideally you want between two and four founders. It would be hard to start with just one. One person would find the moral weight of starting a company hard to bear."[2]

There could be other reasons why a founder chose to be single. In my case, my other friends who were interested in joining as co-founders but, I chose not[include/work them] because they didn't had the same values as I imagine one should. for me, this is more important than their respective abilities.

[1] The 18 Mistakes That Kill Startups [2] How to start a startup