Never thought much one the Lilly Severus Couple my self, not even sure Lilly loved Snape, She considered him her firend until the whole Mudblood thing.
Though here is a thought to concider though, perhaps Snape pushed her away and called her a mudblood to protect her? If he was going to the side of being a death eater... Lilly would be in danger even more so by his side..
It would have worked if it weren't for the prophacy pinning Lilly and James as Harry's parents.
But that is my theory xD Snape loved Lilly, Lilly thought him her firend, Snape alientated her to push her away and out of danger which would have come along with her being close to him. Only to have Lilly become a target anyway by being the mother of James son. So it could be no wonder he is bitter towards James even after his death and Harry to.. even though he does protect him becasue he is Lilly's son to.