I'm sure a lot of money at the end of it would be nice, but I don't think it's just about the money.
I think it's all about solving a problem that you're passionate about. I know this sounds very abstract, but if you think about it, deep deep down, we all are passionate about something. Could be a sport, could be a particular drink, could be cooking, could be Yoga, could be travelling...you get the gist.
When you're passionate about something, I'm sure you know the problems that are existent in your area of interest. Solving that problem is going to help you and the people around you. Boom, you have a start-up.
Am I going to be a saint and say that I'm all holy and say that I'm doing it for the common good, well yes and no. Like I said, a lot of money would be nice, but I think the driving factor is making a difference and changing the world.