For me, at least, it is the extreme belief in oneself to accomplish something. Whether it becomes the next billion dollar startup is second. I am doing it because it is a challenge and not everyone can do it. Its much better than working for someone else and being assigned to do the same tasks over and over again.
The closest mentality to a founder I have seen are those of extreme sports. Especially sports that requires laborious hours and determination to get done. Like those that choose to climb Everest. Its tough and chances of injury is high. But you keep climbing because you believe you can make it to the top. You can't imagine going back down to the mundane life below.