Severus has been hostile to James and Sirius since the day they first met on the Hogwarts Express, constantly humiliated by James’ group, and James even nicknamed him Snivelly.
Despite this, Snape owes his life to James, because Snape, wondering where Remus went during the full moon, was saved by James when he tried to follow Remus on Sirius’ tip, because Remus Lupine is a werewolf, so as not to harm anyone during the full moon. He goes through the secret passage under the Whispering Willow to the Shouting Barracks and completes his transformation there.”
