Imagine how differently things would've been if James and Sirius had befriended Snape on the train. How, with their influence, he'd have been a Gryffindor.
How, in his surprised stupor, he sits beside a sandy brown haired boy who introduced himself as Remus Lupin but only just above a whisper. How he would look across at Lily sitting beside the round faced blonde boy who tripped in his excitement.
How there would've been 5 Marauders instead of 4. How after Christmas break 3rd year and the revelation of his home life, the Potters take him in much like they did with Sirius.
4th year they figure out Lupin's secret and Snape immediately starts working on a potion to help with the pain because there is no cure and it's the least he could do.
In 5th year Snape realizes he and Lily aren't meant to be but their friendship is enough because he has 4 brothers in all but blood and knows James fancies her but hasn't said anything because of Severus.
How after graduation Snape begins his apprenticeship for potions but always makes time for his friends. How, along with Sirius, he kept James from losing it at the wedding.
Think of how proud he was to know he was going to be an "Uncle" before any of the others. How he and Sirius fought over who got to hold Harry first. How he kept in touch through the mirrors after the Potters went into hiding.
Then that fateful Halloween night, he gets to the house just after Sirius and they find them together. Two of his best friends, one his oldest, gone forever.