What I mean is this: I find biology, history, psychology, theology, philosophy, etc. fascinating. I don't perceive them as especially difficult subjects but I love learning about them. I love making connections between different historical events. I love constructing philosophical or theological arguments. I think biology and psychology have a lot of curious information that is fun to learn (especially when studied together). I love these subjects for the mental stimulation they provide and the intellectual games I can play with them.
Math, chemistry, physics, and English on the other hand are something I do for the pride. I love the feeling of looking at a complex diagram with foreign equations (like the one pictured above) and knowing what it means. It's difficult to understand these subjects. They can be abstract and complicated. There is A LOT of high level math. But after working for hours on a single problem with more Greek letters than numbers and 4 pages of mathematical derivations later, coming to the solution is cathartic. Stepping back and seeing the gibberish I just wrote and knowing that I can read the gibberish brings me a pride that motivates me in these tough subjects. In English, writing a well constructed paper is the same way. A sense of accomplishment and pride.
So my learning has been driven by these two forces. Depending on the subject I'm driven by either fascination or pride rather than the attainment of a grade or academic recognition.