The Stalking
Lily had been feeling anxious for a while now, whenever she was away from James and not in the Common Room, she felt eyes fixated on the back of her neck. She looked around behind her but there was never anybody there.
After two weeks of looking both ways before leaving a classroom, she decided that she had had enough; Lily Evans was going to find her stalker if it was the last thing she did.
The feelings of unease and of being constantly watched eventually stopped enough for her to feel secure again so that she was finally ready to tell James, Sirius, Remus and Peter about it. James and Sirius just laughed and called her paranoid, Remus worried that she wasn't getting enough sleeping and Peter just looked around nervously as if to try and spot this mysterious follower.
By the end of the year Lily and the Marauders had completely forgotten about Lily's 'issue' and had moved on; they ran across the lawn joyously after their last exams were over without a care in the world.
Lily Evans may have been over her stalker, but Severus Snape continued to watch her with longing in his eyes as she jumped on James Potter's back while laughing at something Sirius Black had said. Lily was surrounded by friends and had chosen someone other than himself and it hurt. Severus could not understand it. He hated James Potter, but that was no reason to feel hurt, angry maybe, but never hurt. So he consoled himself by watching Lily laughing with happiness, imagining himself with her and picturing his life if he had had her to hold him back.
Stalking Lily may have been wrong, but it kept Severus from completely giving himself to the dark. By stalking Lily Evans for a year, Severus Snape kept enough of his soul in order to help her son.