I'm sure some of these reasons should actually be carefully considered before deciding to go it alone, but I feel that many of them can be safely ignored given one's appetite for risk. After all, if you aren't willing to take chances, why are you trying your hand at a startup in the first place?
tl;dr - Of the common assertions against being a single founder, which are myth/overblown and which are serious concerns?
Yes, that... or his name is Paul Graham! :)
From investors point of view second founder is social proof of the idea and the first person, a much lower chance of entire team getting hit by a bus, and a demonstrable ability to work in teams. They have no downside in modest increase in the number of founders, unlike the first founder who is slashing his pie in half.
You, too, should worry about social proof. If no one wants to hang out with you, maybe your idea is on the wrong side of crazy, and you are the kind of person unable to tell good ideas from bad ideas. Who knows? The thing is, if you have bad judgement, you likely don't know about it, can't know about it.
I would like to know: How you were able to cover aspects of your projects for which you did not have the skills. Not having the skill for design (for example) has been a double blow for me - I don't know what I don't know, so I can't evaluate the scope of projects, nor determine the best use of money and the timing for bringing in external resources.
0) Not important, just interesting. On the moral side: did you reason going solo as to avoid loosing money/control and generally didn't bother or was it just that you didn't find someone acceptable.
1) For the skills you lacked: did you hire someone or tried to learn the skills and figure out how to lessen the importance of this part of the product.
2) Money wise: did you originally planned to bootstrap or wanted to bring in the investors. And how your reasoning about convincing the investors went.
3) Investors: when you finally met real people with money how you convinced them to trust in you
4) Co-founder: What was the primary reason of parting ways with your co-founder
5) Sum up: do you believe that there are 'single-minded' people and they will be better of not trying to find a co-founder at all. That said, it means such people should reasonably adjust their expectations about the product they can pull off alone.
Motivation is easy for me ... I love what I'm doing. Even on bad days. When it's too much though just do something else, or pick some low hanging fruit and knock out some easy achievements. You might have to just make yourself work sometimes, cause if you leave it too long it'll become another dead project/game/whatever.
Who do you talk to? Users. In my case I was fortunate enough to have lots of users including some really great, accomplished people.
You do have to wear every hat and that can make you uncomfortable but you just have to man up and do it, a cofounder's not going to insulate you from everything you're not good at / not comfortable at.
I would love to read about your experiences!