IMO the bar should be much higher for solo founders. If you have 2 people, then it's twice as many resources to check all the various boxes you need (sales, design, technical, etc) - otherwise the solo person has to credibly be an amazing all-rounder (which is rare).

That's the core dynamic IMO... then add on the emotional burden part to that and it makes even more sense.

But then again a bad cofounder fit could actually lead to MORE emotional burden too... so obviously it all comes down to specifics.