I think there are three main reasons that Snape hated James (Harry's father): James was very mean to Snape while at Hogwarts, James ended up marrying Lily (who Snape was in 爱情 with), and James saved his life (Snape was about to enter the shrieking shack on a full moon and, most likely, would have been killed 由 Lupin (in werewolf form) had James not stopped him at the last second).
James was cruel to Snape all the time he was at Hogwarts for no good reason. He was already being treated like crap at home, so he made twice as hard. He was the victim of a prank that nearly cost him his life/humanity, and he probably thought James was the one behind it. And also he took Lily away from him. She was like Lucy was to Benjamin Barker, his reason and his life. Its almost as if he took away all the happiness he could have ever had.
He hates James Potter because of jealousy. Snape and Lilly were 老友记 before going to Hogwarts but their roads took different directions further in school: She was a griffindor and him a slytherin. James always made fun of Snape when they were young and after marrying Lilly, Snape's hatred got even bigger.