Snape loved Lily Evans. He had for most of his life, and that is what dictated his life. When they were kids, they were best friends, but he loved her. I am guessing that at first, about until their second or third year, she loved him too. Snape started becoming more of a Slytherin, and using more dark magic as he grew up, and that (I am guessing) is what stopped Lily from loving him. She didn't pull away fully, as she was still his friend for 2 years before it ended all the way.

Snape never stopped loving her, and even in his death eater days, fancied her. Then when she was going to be killed by Voldemort, he made his decision. He picked her instead of him. One of the most noble things done by a character in the Harry Potter books. And, she was even married. He knew she didn't love him back, yet he still did that for her.

Again, he made a decision for her. To protect Harry. He loved her so much, that his potronus took the same form as hers. A doe. Even though things never worked out between them, his sacrifices he made for her, were very powerful, and changed the course of the books. Lily would have been proud of him.