I like problems that make you slow down. The tangled ones: where the people, the process, the data, and the software are all pulling in slightly different directions. I can happily hyperfocus on that knot until there is a useful way through it.
That also means I get personally attached to the things I build. It does not matter whether a project starts as a tiny personal experiment or a serious piece of client work. If someone is going to use it, I want the details to hold up and the experience to feel like it was made with them in mind.
The part I enjoy most is making something for a person and watching the problem become clearer. Sometimes that means removing friction from an ordinary day. Sometimes it means giving someone a way into technical work without asking them to become an engineer first.