To me at least it was when I took a real analysis course. Very difficult material but once it all 'clicked' I felt like it gave a lot of insight into the fundam

To me at least it was when I took a real analysis course. Very difficult material but once it all 'clicked' I felt like it gave a lot of insight into the fundamentals of math and as a bonus finally offered a rigorous non-hand-wavey explanation of the fundamental theorem of calculus.