[dragonblustar]

Maybe the idea that Snape was secretly in love with Lily is true. If that's the case, he may have come over to Dumbledore and the Order so as to find a way to kill Voldemort as revenge for Lily.

Agreed

I thought Snape had a girlfriend? Nightingale? And wasn't Snape angry at Lily when James and his friends beat him up at the end of book five?

It's been a while since I read the books.

[null and void]

I don't recall Snape ever having a girlfriend.

Snape was mortified at having his boxers waved about while he was still in them.

I think the most mortifying part was that his secret love -Lilly- was there, and worse, she tried to protect him. He lashed out at her to cover his own shame and embarrassment, a very human (and especially teen-aged human!) reaction.