Well, I think what Anne Arthur said about this is exactly right - I certainly can't improve it! She said, roughly, that she resents Rowling privileging a sentimental mother love over the stern love-in-action exemplified by Snape.

Because Snape actually IS very loving. He even loves Harry, and shows it by his actions. But he doesn't *like* him.

It seems that, for Rowling, not liking Harry is the Unforgivable Sin.