In this question one answer includes a quotation from one of the books (and I'm sorry, but as I've said before, mine are on loan and haven't come back, so I'm blind on this!):
Harry Potter: "You mean if he knows about it, then he'll be able to read my mind?"
Severus Snape: "Read it, control it, unhinge it. In the past it was often the Dark Lord's pleasure to invade the minds of his victims, creating visions designed to torture them into madness."
This sounds almost as if Snape knew about this from experience. Did Voldemort torture Snape in this method? Is it possible that some of Snape's emotional issues are a result of this kind of mental manipulation? Is it even possible that part of Snape's unhealthy fixation on Lily is a result of Voldemort experimenting on him?