When I meet founders, I like to know their background. It helps me understand how they arrived at the problem they’re solving. In a recent conversation, a founder told me he’d turned down an opportunity to be one of the first hires at another company. That opportunity was valued at over $1 billion a few years later. When I asked why he’d turned it down, he said he wanted to work on something that keeps him up thinking about it all night. He has no regrets because he isn’t passionate about the problem the other company is solving.

I immediately got what this founder was saying. Starting a company isn’t for the faint of heart. It’s a multiyear journey full of extreme highs and lows. Having passion about what you’re doing is essential. It helps you push through the exhaustion after years of grinding and low points when nothing is going right. Passion is a main ingredient of entrepreneurial success (and of success in life, for that matter).

So, what keeps you up at night?