I think that's because if something's enjoyable, you'll keep on doing it until either it stops being enjoyable or it causes some problem that makes you want to

I think that's because if something's enjoyable, you'll keep on doing it until either it stops being enjoyable or it causes some problem that makes you want to stop despite the enjoyment. By then it's either bad for you and enjoyable or not enjoyable and good for you. So I think your principle is a long term equilibrium position that things end up in, not really true in the short term.