Why is Snape against Harry?

It's overly obvious (I think so anyways) that the reason Snape hates Harry so much is because of the way James treated Snape when they were in school, and then Harry came along as was "So much like his father" that snape couldn't stand it.

That and snape was in debt to Harry's father, and since James was dead, he had no one else to blame except for Harry, so once he paid that back, he was free to hate Harry for the sole fact that he was a Potter.

Shakira you and I are thinking along the same lines. I'm also with smurrfull. I think Snape had a thing for Lily but never went after her, and then James one of the three people he hated got her (the other two are Lupin and Surius).