The biggest reason founders stop working on their startups is that they get demoralized. Some people seem to have unlimited self-generated morale. These almost always succeed. At the other extreme there are people who seem to have no ability to do this; they need a boss to motivate them. In the middle there is a large band of people who have some, but not unlimited, ability to motivate themselves. These can succeed through careful morale management (and some luck).

I'd say about a quarter to a third of founders we fund have never-give-up morale. The rest are mostly in the middle group.

For the people in the middle group, I think the big danger is not knowing what to do next. That's why it's so important to launch early. Once you get something in front of users, they tell you what to do next. Not that you should do exactly what they tell you; you have to guess what they really need. But at least you start to have some data about that.