I've given some thought to the origin of the Snape/James rivalry and the thing that makes the most sense to me, is the idea that maybe it all started as a Snape/Sirius fight. Snape was pretty pro-slytherin and I can see Sirius taking offense to that on day one. If they started fighting and Snape played dirty (which is in character) I bet James would step in and probably show Snape up, thus starting the rivalry.
I think that Snape and Sirius were always more antagonistic towards each other, you’re definitely right! And James would have always defended Sirius, and then of course Snape noticed James’s feelings for Lily and that made his temper flare up even more and so the rivalry snowballed into a dramatic mess.












