Snape's completely selfish, and that's what ultimately makes him the most interesting HP character
Upon my first reading of DH, I have to admit that Snape's 'love' for Lily really annoyed me. That wasn't to say that I was surprised by it. It made some degree of sense, and it had been hinted at. It was more that it seemed to tie everything up a little too neatly, and seemed too much like a last ditch effort to humanise Snape so that Harry could forgive him for all that had passed.

Upon my more recent second reading of the book, I'm still annoyed at the idea that Harry's belief that everything Snape did was for love of Lily could make Harry immediately forgive Snape for every perceived wrong, and could even bring Harry to name his kid after Snape (though don't get me started on how morbid it is for all the next generation to be named after dead people).

However, I've come to accept the unrequited Snape/Lily plotline on its own. I don't believe for a second that Snape was truly in love with Lily. To really love someone is to care for them regardless of whether you'll be benefitted by that emotion. Snape didn't love Lily. He was obsessed with her. Every feeling he ever had for her and every action he took in relation to her was selfishly motivated.

In fact, as I came to realise upon re-reading DH, almost everything Snape does in canon is almost purely selfish. And that just made me love him more.

It's almost confusing to track Snape's progess through the books as (from Harry's POV) evil, then good, then evil, then good. Snape is neither evil nor good. But despite appearances, he's neither Dumbledore's man nor Voldemort's. Though it seems that he's on one side one second and another the next (is there such thing as a quintuple agent?), in the end he's not really on anyone's side but his own. And, to me, there's nothing more interesting than a character that does both good and terrible things because they are completely selfishly driven.

Joining Voldemort was for Snape's own benefit. Trying to get both Voldemort and Dumbledore to save Lily's life was for Snape's benefit (he wanted to possess her, and failing that he still wanted her to stay alive and well because it would devastate him for the possibility that she might one day be his to vanish entirely). Snape spied for Dumbledore and protected Harry because it was the only way to keep whatever connection he thought he had with Lily alive.

Snape does the right thing for the wrong reasons (e.g. trying to save Lily's life, helping Harry, etc). But then, he also does the wrong thing for the right reasons (e.g. killing Dumbledore). In fact, in the entire series, Dumbledore's murder may be the only thing he does regardless of the fact that it will in no way benefit him personally. Even though I also find it very sad, it also tickles me a little that murdering the only man who ever really gave him a chance was Snape's most selfless act.

It makes me somewhat glad that Snape was killed in such a pointless way (if you actually believe that he died, which most of his fans seem to deny to the death). I don't think I could have bought it if he'd selflessly sacrificed himself to save Harry or so that the wizarding world could go on without Voldemort. It would have been too Gryffindor. And hey, that's clearly what Harry's for, right? ;-)