Have you ever studied from a subject, realized that you were lacking in some prerequisites, then started reading up on the prerequisites, and then realized that you have become more interested in the prerequisite subject(s) than the original subject? I have. Years ago, I was very interested in general relativity, but realized I needed to learn more differential geometry to study it. So I started reading differential geometry, and then fell in love with differential geometry. Now my main interests are differential geometry and differential topology, and I don't think about general relativity as much as I used to. Also, a long time ago I was studying topology, but I realized I needed to study more set theory. Then I became very interested in set theory (though I still like topology very much now).