My only successful product to this date is an app I built because my wife asked me to. It is in an non-technical domain which I knew nothing about. I thought it was rather non-promising, but, since it was a pet-peeve of hers, I gave it a try.
It was an awesome (and very bonding) experience - she explained me the problem(s), and I tried to simplify and structure it (didn't think gardening could be so complicated). Both of us were in their respective element, and from back and forth an app was forged.
To this day I only half-jokingly call her my product manager. The app has brought in 5 digits last year and is rising.
Last week, she briefly mentioned another problem, in another hobby domain of hers...