Why would Snape ask Harry for his take on Snape's past? One of the underlying premises of the story is that the smarter characters (possibly with the exception of Hermione) always have a deeper plan. Did Snape actually expect to get good advice? To be told that all his choices were correct? To have a reason to be angry at Harry?

I also wonder why Snape got offended. Harry's answers were extremely supportive of Snape's situation back then: which makes me think Snape wasn't really offended, just pretending to be. Maybe the whole point was just for Snape to tell Harry the unpleasant truth about his parents in an emotionally powerful way, as a way of getting back at Harry because of his parents.