Snape and Evans were inseparable, then they got in a fight and he stayed outside the common room until she came out.
Do you think the Slytherins didn't notice? That he was literally groveling at the Gryffindor Dorms and crying his eyes out to apologise to a girl he called a mudblood? You think the Marauders didn't notice?
He completely humiliated himself in front of the entire school, to be told to go away.
And then he did. No questions. No contact.
That was their 5th year.
5 years later Snape hears Lily's name and asks the Dark Lord to spare her. He does not. He asks Dumbledore to save her. He does not. He sobs over her body because they never had a chance to grow up and make up, like people do.
Maybe not this generation, but certainly in mine and definitely in the ones before mine. At some point around 20 you look around at all the people you know and wonder who you're going to be around in old age. Then you realize it's probably the crabby guy in the corner, knowing your luck, and you just kind of surrender, and so do they. By the time you hit 50 you're punching each other in the shoulder over it and shocking your grandchildren with the story.
So, Snape's fate is sealed. There is nothing else left of his childhood to draw upon. But he does make a vow to protect Harry.
Then Harry shows up looking like James, who Snape blames for getting Lily knocked up and putting her in this position to begin with and he snaps.
Do I think Snape was in love with Lily at the end of the books?
Honestly, I don't think so. I think he had a deep loyalty that is absolutely love, but in the same way that if I heard something happened to any of the kids I grew up with were killed and their child needed protecting. Even for the people I thought were awful.
However if that child looked like their parent did at that age and had some of the same tendencies I'd be sitting on my hands so I wasn't continuously slapping them on the back of the head.
And I don't think she was the last person, or only person, Snape ever loved. I think he chose to repress those feelings whenever they came up so no one near him would ever have their safety compromised. I also think he got 'flashback' love when he had to deal with Harry and his eyes every day, but I doubt it was a real thing. And I do honestly think Harry was triggering flashbacks.
People act like he's some giant romantic hero, or some obsessed psycho, but I don't think he was any of these. He's like a seriously flawed knight that manages to do one good thing before he's killed, and manages to tell some kid his life story as a cautionary tale.
I also think the doe thing was out of guilt, not love.
And when Snape broke into Harry's mind he didn't sneer or lord over him. He knew who Petunia was. When he saw her laughing as Harry was terrified and run up a tree the first thing he did was ask Harry who the owner of the dog was. I think it truly shocked him.