I have other posts that I've been trying to digest and comment on, but this one stole my attention for the moment.
Maybe JK had put these particular memories of Snape's in because they show Snape's human side. It allows us a chance to think "oh, I see" and maybe even feel a little sorry for him. But it's also showing us some rather vivid parallels between Snape and Harry.
In Snape's memory of his parents, Dad raged and Mom cowered; in Harry's his mom threw her herself in the face of danger at risk of her life.
In Snape's memory, he struggled with a bucking broomstick and was rewarded with mocking laughter; Harry fought a bucking broomstick and was applauded, patted on the back for his ability to cope.
Maybe it doesn't mean what I think it means, but it sure looks like Snape is holding onto memories that smack of contrast between himself and Harry. Fuel for the fire, so to speak. I don't know if Snape is feeling any sense of injustice at these, like he resents that Harry's mom was brave where his was a coward, or if he's playing these up to intensify an emotion he needs for another issue (see Snape the Death Stopper for more on that issue). Any thoughts?