The number one and two causes of startup failure in my experience are fights between the founders and VCs forcing the startup to do the wrong thing (which often causes fights between the founders with one saying "we know the market better than those guys" and the other saying "but think of the signal if we don't pivot into widgets for baby monkeys and they decline to pro-rata our B round???!")
But the bottom line is, if you don't know someone you can cofound with, you shouldn't jeopardize the future of the company by just picking one. I don't think you can pick a good co-founder with less than a year of experience with them. Founding a company strains previously good relationships by greatly raising the stakes for every decision and disagreement, compared to your previous relationship with each other. PG forcing Dropbox's founder to get a cofounder seems shockingly dangerous (unless that cofounder was really always going to be number 2 in authority).
If you know someone you've worked with for 4-6 years and they work their butts off, then that's a great potential co-founder. I had one of those, but we got divorced. :-) Still on good terms, but I am not going to start a company with him.
But if you don't have anyone like that- you're ADDING relationship risk to your Startup Risk. And you're unlikely to be reducing your startup risk by the same amount. That co-founder could be hired with a significant opportunity to earn equity, at about the same time, and be just as successful at reducing startup risk without adding the relationship risk by being an "employee". The difference is in control.
So, my plan is to be the Founder, and then get a set of co-founders who get founder stock[1] but don't have the authority to overrule me, even if they all gang up (eg not an equal equity split). Hopefully this will be acceptable to TechStars. This will show others are willing to work on my idea at least- which is the claimed reason for a cofounder cause it's early social proof (also a really silly signal).
[1] I don't believe in multiple classes of stock. A dollar put in by an employee from under market salary is just as valuable as a dollar put in by a VC firm, and the employee is taking more risk- they can only "invest" in one job at a time.