I was reading highlander and weiss kreuz fic when half my friends got into harry potter. I was reluctant, b/c it was so popular, but saw one of the films and thought it was cute enough, so I let them suggest some of the fics they were reading. Consequently, I started with remus/sirius and snape/hermione fics. I liked both pairings well enough, and still do, but I've never really liked hermione, and the remus/sirius stories were just missing something.
So I thought to myself; I like snape, and I like slash, who could I pair snape with? Well, harry's pretty malleable as a character -- I bet his fic persona is better than in the books.
Then I googled snape/harry slash, found an archive (ink stained fingers) and never looked back. I don't recall the first hp or snarry fics I read though...