One reason could be jealousy. Snape might be jealous of James' popularity and charm. James, on the other hand, could be jealous of Snape's close friendship with Lily in their early years. Another reason is their different personalities. James was outgoing and bold, while Snape was more introverted and brooding. Lily might be attracted to different aspects of each of them, which could lead to the fight as they both compete for her attention.
In the original 'Harry Potter' series, it seemed that Lily had deep feelings for James in the end as she married him. But in fanfiction, it can be very different. It could be argued that Lily had a complex relationship with Snape too. Maybe she had some level of love for Snape in the past, like a childhood friendship - type of love. However, in the context of this fanfiction, it's hard to say for sure who she really loved as it depends on how the author of the fanfiction has written their relationship.
Perhaps in the fanfiction, James is seen as more accepting of Lily's friends and her social circle. Snape, on the other hand, might be associated with a darker crowd or have some beliefs that Lily doesn't agree with. For example, if Snape is too involved with the Dark Arts in the fanfiction, it could be a turn - off for Lily.
It could make Snape more bitter. Since he lost Lily to James, he might become even more withdrawn and focused on his own studies or other solitary pursuits. This could further isolate him from others and fuel his resentment towards James and his group.
In many fanfictions, Snape's jealousy of James and Lily can be deeply rooted in his unrequited love for Lily. Snape had a complex relationship with Lily from their childhood. He loved her deeply, but she fell in love with James. This led to his intense jealousy. His envy towards James was not just about Lily's love, but also about James' popularity and charm which Snape felt he lacked.
Love and jealousy are very common themes. Snape's unrequited love for Lily while she is with James creates a lot of jealousy. It often drives the plot as Snape's actions are influenced by his feelings.
Unrequited love is a very common theme. Snape's love for Lily that was never fully reciprocated is often explored in depth. It adds a lot of emotional depth to the stories.
Guilt and redemption can be a theme too. Snape's actions were often driven by his love for Lily and his guilt over her death. Fanfictions may expand on this, showing how he tries to make up for it in different ways. Also, the theme of loyalty - Snape's loyalty to Lily even after her death is a powerful concept that fanfictions often explore, sometimes by creating scenarios where Lily somehow comes back or is still present in a way that tests Snape's loyalty.
Snape's view of James might not change much at first. He has such deep - seated hatred for James due to their past. But if the fanfiction shows James in a more positive or sympathetic light, he may start to question his long - held beliefs about him, just a little bit.
Often, his jealousy is manifested in his actions towards James. He might try to outdo James in academic or magical feats, all because he wants to prove that he is better than James, especially in Lily's eyes. And when it comes to Lily, he may be secretly longing for her attention while trying to hide his feelings because he knows she is with James.