For me it was the lack of nice typography/typesetting of Math content with other programs/philosophies (20 years ago...)
I started my studies of Physics and Mathematics at the University of Freiburg in Germany in 1996. In my first year I had the plan of typing some of my notes from the lectures to have it more readable (to others and me too ;-))
I first used MS Works/Word (there was no Formula editor then, if I remember correctly) and had to copy over symbols from strange fonts I found on a shareware CD and was still frustrated that my texts looked not as nice as the worksheets and problem sheets from university we had solve each week in order to credits.
I asked on of assistant of the professors how they were done and his short answer was: "I use LaTeX -- go to our computer pools, it's installed there."
Fortunately, I had an account and went there and tried it, using strange search engines to look after LaTeX (providing other results that I did not searched for ;-)) -- on a very early version Linux, however, my knowledge of both LaTeX and Linux was limited, I bought (!!!!) a 5 CD - Volume snapshot of the CTAN - archive of 1997 for about 40 DM, which would be about 20 € nowadays.
I went home and installed a LaTeX version which I can't rememberhow it was named (emTeX, perhaps) and did my first trials on my last computer that was based on MS Windows only, I had no internet connection at that time, so getting information was much more difficult (especially, there was no TeX.SE or latex-community.org in order to look for advises and solutions)
I found LaTeX fun and the results were much of my liking and definitely better than what I generated with WORD etc. Of course, not everything was typographically beautiful.
Heading towards my Diploma thesis I became tutor of younger students and had to assist professors, it was then my 'job' to provide worksheets with LaTeX, Lab course reports and seminar talks were typeset with LaTeX (that was before
beamer appeared) and my final report for gaining admission to the Diploma level before starting the thesis itself was the first 'major' work I wrote.
Of course, my Diploma thesis was written in LaTeX (using
book.cls) and all papers/talks later on as well as my PhD thesis.
Graduating a third time with Teacher Seminar I wrote my 3rd thesis again with LaTeX and nowadays 99.9% of my work sheets, formularies etc. and my presentations are done with LaTeX and I get many praises by my students/pupils for providing readable and comfortable texts to them, compared to other teachers that still glue things together before using the copier ...
Nowadays I have gained some knowledge about the internals of TeX etc., but I am no core developer, of course, although I was able to ship some packages to CTAN already.