I'm learning some different ways to say "I like", and to my mind right now, keen on, fond of, and like are pretty much the same. However, since I almost never said keen on and fond of before, I'm afraid that they might not be appropriate in certain contexts. For example, maybe you only use them with objects and hobbies, not people or phenomena, to name but a few.
Is this one of them? In which situations would you advise not using "keen on" and "fond of" to mean "like"?