Conventional wisdom says the idea comes to you. A frustration/problem of your own, or at least a problem you care about, if it's not your own. The reason is that if you don't identify with and care about something, you'll quit at the first roadblock. And there will be plenty of roadblocks, for any startup idea.

But plenty of people have succeeded by actively looking for a startup idea. The founders of Homejoy iterated through 12 startup ideas, before settling on Homejoy. I know successful entrepreneurs that have started meaningful businesses by looking on Google Trends. That's the least organic approach. But it works, if you're a good executor.

So, it's OK to take a pro-active approach, in my opinion.

In the end, what makes a startup successful (aka sustainable) isn't where the idea came from. It's down to one thing: does it make something people want to pay money for?

In healthy cases, that implies you're solving a problem that helps someone achieve an important goal of their own.

And that is the goal of entrepreneurship, in my opinion. To solve people's problems.

If you really level up, and become a true master, you get to solve people's individual problems and society's problems. Like Elon Musk, trying to help the world transition to clean energy.

So in a nutshell, proactively look for people's most difficult problems, and ways to make the world a better place. It sounds cheesy, but that's a good way to make money, which in turns helps your startup grow.