This is what I'm thinking... some spoilers for those who haven't read the books.
I think that Snape will realize that its obvious that he treats Harry because of how Harry's father treated Snape. In book 6 Harry will begin to respect Snape for who he is and what Snape can teach him to defend himself. By respecting Snape, Snape will realize that Harry is ready to learn from him and will have to patience to teach Harry (of course persuaded by Dumbledore).
I just see that Snape realizing that Harry is the answer to saving the Wizardry world sacrifices himself to protect Harry from death.
WHAT! HOW DARWE YOU! HE IS MY FAVERITE CHARACTER. HE IS THE EYS AND EARS OF HOGWARTS! HARRY AND CO. WOULDN'T EVEN BE ALIVE IF IT WASN'T FOR HIM!HE GHAS DONE SOOO MUCH! HE HAS HAD A HARD LIFE! EVERY SKOOL HAS A TEACHER LIKE HIM. YOU SHOULD BE ASHAMED OF YOURSELF! starts throughing tomatoes and other rotted vegs to icy
Why did Snape switch sides?
I'm guessing maybe Snape fell in love with some one when he was a Death Eater, but Voldemort had her killed (for who knows what reason), so Snape went after Voldemort himself to avenge her (or his) death but realized that he could not harm Voldemort in any way, so he did the only other thing he could do, even though it meant he had to join forces with his school bullies-he became a spy for Dumbledore ( after all people, love CAN move mountains). And maybe thats why he doesn't have any girl friend (or a boy friend), and maybe thats why he didn't get married, and thats how Voldemort knows that Snape was the spy, because he initiated in her (or his) killings-hey, this all makes sense!
Now, it might sound a bit farfetched to some of you, but honestly, can you come up a better idea as to why Snape might wanna switch sides and help the people he hated most through out his whole life?