he's never given any indication of having any feelings towards Snape except dislike verging onto hatred.

But that's the First Law of Slash: All intensity is sexual intensity.

(I once saw a cartoon where Popeye and Bluto were at a party, holding hands, and one of them was telling one of the other partygoers, "We finally figured out where all that anger was coming from.")

I find Harry/Snape quite compelling; it isn't so much that the canon supports it, as that it does something for me. I think it's all that loneliness -- it's very satisfying to take two characters with such depths of emotional emptiness and give them both what they need in one fell swoop.