This Founder's Obsessive Music Habit Spun Into Three Different BusinessesDon MacKinnon's love of mixtapes helped him start multiple successful ventures.

I have built two companies and am now growing my third,Hark Audio. Often, people wonder how serial entrepreneurs like me continually come up with new ideas, and my answer is this:Starting a businessdoesn't always mean starting from scratch. If you have a powerful, foundational insight about something people love, you can make a career out of building concepts on top of it.

Dennis Crowley did something like this. His obsession with location-based social media led him to form Dodgeball, which he sold to Google, and then to cofound Foursquare. Similarly, Paul Davison and Rohan Seth were passionate about finding new and more intimate ways of connecting people, which led to multiple failed startups and then one that hit: Clubhouse.