|My dear, you will be pleased to hear that I do not love you like Snape loves Lily.|
Snape was Lily's friend from the age of 10. He had plenty of chance to be her friend and to develop that friendship but he made some bad choices. The bad choices started with him insulting her sister, continued with him disliking her boyfriend (although that wasn't all his fault) and finished with him getting involved with the death eaters.
I don't love you like Neville loves Herbology, or like Hermione loves books, either. The difference is that our love is mutual.