You know, it really depends on the research problem. Sometimes it is because I know that a certain piece of work is very important. People are relying on *me* to get it done and I don’t want to let them down.
Sometimes it is because the problem is a challenge. It becomes a bit of a competition and I don’t want to let the problem win! I will then keep thinking about something until I crack it.
Often I keep at certain areas of research because as soon as I answer one question, it raises new ones and I have to keep going to solve those too. It’s a bit like trying to find out what happens next.
Also, sometimes the research can be fun. And, if you enjoy something, you keep at it!