“Learning takes wings when it’s related to what is passing in the student’s inner life.” So says Thornton Wilder in a wonderful book, Theophilus North, which will teach you why its hero could have been called Amadeus or Gottlieb, and what is creative kindness.

How can locally integrable functions be related to anyone’s inner life?

Let us reformulate the question in what seems to me the spirit in which it was asked: Why are we going through material which is hard and demanding, not particularly interesting, and irrelevant to anything of importance in my future career? Is this not the way to regurgitation for a qualification?

I think you should see by now the outlines of my answer to the sad student. Every bit of material we teach, in mathematics and English, in physics and philosophy (as opposed to engineering which is partly vocational training, and certainly as opposed to various non-disciplines, such as Hospitality Science, Sport Science, Media Studies — to name just the least litigious — in which no thinking ever happens even by chance) is an invitation to think. OK, $e^z$ is never zero, but is there any other complex number that this function cannot hit for some $z \in \mathbb{C}$? What are the implications of the fact that for some function $f$ from the unit interval into itself, $x_2=f(x_1)$, $x_3=f(x_2)$, $x_1=f(x_3)$?