Uhmmm… I have been thinking for quite a while about this subject. I started to read this thread when it was about five posts long.
[quote=Laurette]Louise, even though I share your disgust for James as he is presented in the books, I actually like Lily. She doesn't remind me of Ginny, but of a perkier Hermione[/quote]…
I am of the same opinion. I do not even imagine Snape attracted to a girl like Ginny who he could find snogging somebody at every corner… (*cough* sorry

).
The thing is I do not think Lily betrayed him. The marauders must have found out about the
levicorpus spell for some other way, maybe as Ygraine said Snape used it on someone. Because I think Lily maintain her distance for good, that it was real her dislike for James´ arrogance.
LilyPotter --- Although I am not throwing at you my books, I throw at you any other thing that crosses me. James adorable as "Bad-boy"? for Christ´s sake!

(Just kidding!) / Now, seriously I do not like James he is so argh! The positive is that I do not feel jealous towards you for having to share Snape even more. Lol!
So here it is the combination of my first thoughts about it combined with all I´ve been reading here:
1. Since James and Snape weren´t very friendly with each other, this was not the first time Snape was humiliated in public.
2. From HBP we know Lily and Snape have a lot of time to get to know each other, as somebody point in the thread too, because they were very good students.
Snape meets this smart, beautiful and noble girl who is Lily and starts to feel something for her, but we do not have any information about what could have happened. All we know is that Snape is a really proud boy and not exactly… I mean I can´t imagine Snape accompanied by a girl in a scene of the kind of Harry with Cho in Madam Puddiefoot´s place.
3. From HBP we know that what Snape says to Lily doesn't have any meaning since he was not a pure blood either (although here you can arguee that most of the DEs are not exactly pure blood). I think he said so in an outrage moment without second thoughts. I think Snape said something as absurd as that because he was feeling worse by the fact that she was helping him.
So in the scene of the pensive I believe Snape felt really bad and more anger than usual, because Lily was there and saw him so defenseless. He must have felt terrible thinking the girl he liked saw him in such situation. And moreover she probably believed him so weak, she thought necessary to interfere and to try to rescue him.
4. And finally Snape witness how Lily can affect James temper. In the "Snape´s worst memory" chapter.
For Snape to realize that his worst enemy acts pretty different when Lily was around (and that she can upset James in such way) must have been even more disturbing.
[quote=Laurette]I'm assuming Lily had reason to choose James, and it was all wonderful that she did. This left Snape hating one part of Harry and experiencing extreme pain because of the other part of Harry. Is it possible that he feels a fatherly feeling for him? This sounds way off the wall, I know, but if he saw himself as a possible partner for Lily, he could have been her son's father. Weird, loopy little theory, but does anyone else see this? So he cares for Harry (unbeknownst to himself of course) by trying to sneer the James out of him.[/quote]
Oh my gosh! The part of "he could have been her son´s father" make me shiver. Even so I agree, Snape do care about Harry and has protected him more than once. All his means attitudes could be the expression of his opposing feelings. Could you imagine to have to be so close to the son of whom you believed was "the love of your life" and the same who ended next to someone whom you despise with all your insides?
I remember the first occlumency class, Snape was mean as always but in fact he said many useful things to Harry. Snape is like a really twisted and authoritarian father sometimes. With all his paraphernalia of
I treat you with no mercy because the Dark Lord is not going to have it either! Or as someone said before "what do not kill you makes you stronger" something like that.
[quote]Originally I thought that that particular memory was in the pensieve because Snape was viewing it... as in, over and over and over again (...) I felt that Snape was agonizing over it.[/quote]
[quote]Snape is pulling them from his brain because he doesn't want Harry to see it.[/quote]
I think it is a mix of both quotes. Definitely Snape had to be viewing constantly that precise memory because other way he just had forgot that was valuable information for a kid that can use that fact as vengeance. We had to have in mind, also, that Snape take Harry out of the memory is not sure the memory ended there. Almost certainly Snape placed in the pensive the answer to our hypothesis.
[quote=Ygraine] Snape, and Lily living near each other?[/quote]
You know Helen I like this idea because it opens a new possibility (at least for me).
When Harry was in class of Occlumency, one time he protected himself and was capable to see some of Snape´s childhood memories. That time I thought Snape lived very hard moments, but you see I always pictured him alone (a lonely kid as he is a lonely guy). The thing is if Lily lived close to him and they maintained a friendship, he could have shared his emotional burden.
I envision Snape and Lily as really close friends, maybe Snape´s only friend.
Place yourselves in a situation in which you believe your friend betray you. For some reason she slip out one of your secrets (I know, I know this contradicts what I said in my first lines). You have trust issues and even more you are such a proud person that you just became really bitter and never confront the situation in an attempt to find out what really happened. That does not mean that you do not love you friend anymore, it just that you do not feel in the mood to stay close to him/her any longer.
I do not know how to explain this. It is just that I have the feeling that between Snape and Lily was a big unresolved problem: a misunderstanding or something. Lily looked quite surprise when Snape call her names.
Arghhhh! I need to think it deeply... My head have like a thousand data going around…