There are many reasons. James Potter and Severus Snape both loved the same person, Lily Evans.
Lily hated James but was friends with Snape until her fifth year. She began to date James in her seventh year after he grew up a little. However, both were very jealous of each other.
James also hated Snape because he was a Slytherin and supported the death eaters.
Severus Snape He had known Lily before they both went to Hogwarts
Professor Severus Snape! Also, Dolores Umbridge (probably less than Snape, although she was quite evil).
Tobias Snape is Severus Snape's father.
Severus Snape teaches Potions class.
Not really. Harry Potter hated Severus Snape until after his death when his true nature was revealed.
The Patronus that led Harry to the sword was Snape's Patronus.
In Order of the Phoenix, Professor Snape teaches Harry Occlumency and Harry's least favourite, Potions.
Severus Snape was headmaster in book seven: Harry Potter and the Deathly Hallows.
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Severus Snape took many points away from Gryffindor throughout the years. He Favorited Slytherin house and had a grudge against Harry's father, so took it out on Harry.
Professor Severus Snape is a major character throughout the series. He is the Potions Master at Hogwarts and takes an immediate dislike to Harry due to his likeness to his father, James, who used to bully Snape at school. Despite this, Snape does try to protect Harry and saves his life from Professor Quirrell when he tries to curse him off his broomstick.
Alan Rickman plays Severus Snape in the Harry Potter movies.