Solve Your Own Problems
Some of the best things people build start as a fix for an annoyance they couldn't shake. You run into the same wall over and over, patch it badly, and eventually get fed up enough to make something that actually works. That itch is where a lot of good work comes from.
When you build for yourself first, you already know the problem cold. There's no guesswork about whether anyone cares, no survey to run, no committee deciding what matters. You are the customer, and you're a demanding one. If the thing you make is clumsy, you feel it every day, so you keep sharpening it until the friction is gone.
The opposite approach gets people into trouble. They dream up a product for some imagined stranger, guess at what that stranger wants, and pour months into a solution for a pain they've never felt. It usually shows. The edges are wrong, the priorities are off, and the whole thing has a secondhand quality to it.
There's a quieter benefit too: motivation holds up. Chasing a market you don't understand burns out fast once the novelty fades. But fixing something that bugs you personally keeps paying you back the whole time you're working on it, because every improvement makes your own life a little easier.
And odds are you're not the only one stuck on that particular thing. The problems that gnaw at you tend to gnaw at plenty of other people who never bothered to do anything about it. So scratch your own itch. Build the tool you wish existed, the one you'd reach for tomorrow morning. Start there and see where it goes.