It seems obvious to me that there are cases (not only involving learning) in which people quite reasonably do things neither for your reason 1 nor for your reason 2. For instance, people often do things in order to be paid, even though they don't enjoy them all that much for their own sake and they don't do much to build skills. People do things to impress other people. People do things because they consider they have a moral obligation to do them. Etc.

The idea of "learning for its own sake" may be a puzzle even for a steelmanned version of your dichotomy -- e.g., you'd think people do things either because they directly enjoy them or because they are a means to some directly enjoyable end (though I think even that is too narrow, actually) -- but as it stands there are just waaaaay too many obvious exceptions to your dichotomy for the fact that something doesn't fit it to be puzzling at all.

For what it's worth, I think a leading reason for the particular phenomenon of learning-for-its-own-sake is simply that these people have spent years in an education system that consistently praises and rewards them for learning. It would be hard for them not to emerge with a feeling that learning is good for its own sake. (Which it might be -- or it might be consistently valuable in ways that would be hard to predict in advance -- and those may be among the reasons why the education system is that way.)