It's love for me, but I'm very unusual. I'm long-term ill (progressive neurological disease), and can't work any more. So my PhD is of no financial benefit. The first time I signed up for a PhD (over a decade ago, science, full-time) it was because I wanted to be a lecturer. No incentive of money at all. I think anyone thinking of going into higher education who's after lots of money is a fool! The second time (history, part-time) it was sheer love of the topic.
I really think love has to be the stronger reason. A PhD is a terrible slog at times, and can be extremely difficult to stick to the end of. If you're just in it for the money there are easier ways. For example I'm not convinced in the current economic climate that it is the way to higher pay.