In my opinion, there wasn't a good reason. It started when they were in the train and the way Snape acted... I suppose it represented everything that James disliked about Slytherin. All Snape alisema was "you better be in Slytherin" to Lilly and James heard that and put down the house. Snape retaliated kwa dissing Gryffindor and then James called him "Snivellus" and Lilly and Snape left the train compartment. Seriously, that seemed to be the first incident. Very lame.
But ever since then they hated each other and whenever James and the Marauders bullied him (for the reason of "because he exists") Snape fought back and was never nice to them. So it just continued.
Later on, it was probably 'cause James like Lilly and Snape liked Lilly and Snape and Lilly were Marafiki and Lilly was always defending the opposite person with one of them... Jealousy. Then Snape got into the Dark Arts and James hated the Dark Arts and Snape seemed zaidi like a Slytherin stereotype.
And what is the difference between a child who has everithing and one who has nothing! The problem existed from the very beginning: James was loved kwa his parents he had a lot of friends;Snape's parents didn't care about him, his only upendo was Lily-but she left him for James! How to live with a broken moyo and not to hate the one who aliiba the upendo of your life?!
Severus was always in upendo with Lily Potter since they were kids but Lily never saw Severus in that way he was just a friend and then, when they go to Hogwarts James and Lily fell in upendo (not this fast, of course)... Severus doesn´t like James cause he took Lily from him
from the beggining snape and lilly were friends james, suris, lupin and wormtail made fun of him (well lupin did not) since when they first meet because he wanted lilly to be in slytharin they heard what he alisema and also because they wanted fun then lilly stoped being his friend and started to tarehe james simple
It just happend when James alisema He wanted to be in Griffindor and Snape snorted.and alisema he wanted to be in Slytherin.And well they never really liked each other and Lily was alway hanging out with Snape not James.And Griffindors and Slytherins never liked each other!!