I would say the friend component often overlaps with people you already know are really great to work with. Many of my best friends are people who I've had the most positive experiences working with on the job. One of the most important things to have in co-founders (especially bootstrapped ones, I suppose) is trust in them to stick it out and not let you down. While it may be risky to go into a difficult situation with friends, at least you're much more sure you can probably rely on them and know what they're capable of.