I can relate to this. My major was EE and I had similar experiences, but kind of liked diffeq. Fast forward some 25+ years later, I spend all day coding and look to my old math and some new comp sci books for fun. I keep an engineering notepad with different sections in it so I can just open it up and attack problems such as concrete math problems from graham knuth and patashnik or physics problems. Even though I code through business problems on a daily basis using X framework and Y API, I find the rigor involved is not as satisfying as struggling through finding the closed form of some equation. I want to solve a deeper problem, if that makes any sense.

It isn't the answer that I seek in doing this; it's the desire to know why the answer is what it is. I think I have become more interested in learning far after graduating from university.