Now that I have probably angered a bunch of Snape fans, let me explain.
I love Snape and his death broke my heart, and of course I wanted him to live! I wanted him to live because I felt he deserved better than dying the way he did, never having a chance to really make his own life, as he was always bound to his dead love, his guilt, mistakes, and his masters.
However when I really think about it in a canon sense, and more detached from my Snape-love, it really makes perfect sense to me that he died and it seems right that he did.
If Snape had survived the war, what sort of life would he have had after? Fanfictions deal with this a lot, and in many varying ways. I love to read about "Snape survived" fanfictions because the possibilities are interesting and endless. Does Snape get away from it all in a quiet cottage in the Orkneys? Does he stay at Hogwarts and take up his old post as Potions Master? Does he spend the rest of his life with effects from Nagini's bite? Does he finally move on, and by some strange fluke, a relationship begins to build with someone? Does he fall in love again, and finally get closure to his past with Lily? Does he have children, and become a wonderful father? Does he start a mail order potions service? Does he serve time in Azkaban? Is he pardoned for the Mark on his arm, and lauded a hero?
The list is really endless of 'what ifs'. But when canon comes into play, I really don't think any of those good things would have happened, or could have happened.
I do like reading other Snape pairings, but the TRUTH is that Snape loved Lily, and he would ALWAYS love Lily. I don't think he would ever stop, and I think allowing himself to even have a hint of romantic feelings to someone else would have felt like a betrayal to her. Snape does not half-ass things, he does them head on, fervently, with obsession. Snape's torch for Lily is one big ass torch and I don't think he would allow anything to put the smallest damper on it.
If Snape had survived, what would he have really done after the war? He spent his life first serving Voldemort, Dumbledore, and his lost love. He was always driven and chained by his love, passion, and guilt. Snape and Lily is not a pretty love story at all, and Snape is a very tragic figure. We like to emphasize how hard Snape loved, how fully he loved, but it really is a sad and miserable sort of love. As a result of his actions he had driven most likely the one person who had ever cared for him away. If that wasn't enough that he ruined a relationship that meant so much to him, his actions later result in her death. In my mind Snape doesn't ever think to himself 'Voldemort killed Lily' I feel like he would always think 'I KILLED LILY'. Like I said, Snape does something, he does not half ass it. I think this would include self-blame. When he was with Dumbledore after her death, he was practically suicidal.
After the war Snape would no longer be indebted to a master, and he would have served his purpose to protect Harry. What would there be left for Snape to do? He had basically spent his entire life since the age of nine (minus the part in between where he lost his head, called her a Mudblood, and got a ridiculous tattoo) being a willing slave to his love for Lily. When it was all done, what was really left? What purpose would Snape's life now have? He never seemed to like teaching much, so I doubt he would have found much in returning to teach, unless it was just routine and familiarity...which are both good things.
I think his life would be strange to him having nothing to really be dedicated to any longer. I don't really think he would have felt closure, as even if Lily forgave him, I really don't think Snape would ever find it in himself to forgive himself. He is a man who holds grudges, and don't think he makes an exception for himself. His life would be lonely, because he would not find someone else to share it with. I think he would just continue to be miserable, lonely, and dissatisfied with his life, and that really saddens me.
Snape's death was good for him. His life was the ultimate price he could have given for his love, and I think he had no problem dying for Lily. I think if needed, he would have died for her over, and over, and over again. I don't think anything else would have ever been enough, in his mind. No amount of service, dedication, protection, spying, risk-taking, years, decades, just none of it would have ever been enough in his mind, I think.
I think Snape also finds a kind of peace and closure in his death: and ability to maybe finally move on.
Why do I say this? Here's why.
Throughout all the books Snape loathes Harry. Over and over again it is because Harry is like his father. Always, Harry is like James. I think Snape asking Harry to look at him in the end, is not just a sadly romantic sort of thing that JKR stuck in there. I think it is a very deep symbol. When has Snape ever looked past the James in Harry? As Snape has surrendered his most private memories to Harry, as Snape is dying, he is finally able to look past his grudges, his anger, his bitterness, his pain, and to see the good in Harry...to see how like his mother he is. I think this moment is so powerful for both Snape and Harry, because in that gaze they both come to terms with who the other really is.
I think Snape has finally found a peace, and that maybe he is ready to go, knowing he has done all he can and there will be no more crucifying himself for his mistakes.
It is done.