Dean leaves Sam at a critical time (could be right after confessing feelings for each other, or some other ‘bad time’ to leave). When he leaves, he is deliberately either cruel (verbally) or in some other way makes it so that Sam would never want to follow/go after him.

Ten years (or some other period) later, Dean comes back to mend fences. It turns out he had a very good reason for leaving under the circumstances that he did. Maybe he was blackmailed and did it to save Sam. Maybe being around him (Dean) at that time would have been dangerous to Sam. Whatever his reason, he did his best to make sure his stubborn little brother would never seek him out. The threat or problem that caused him to leave is now over - or better yet - it is threatening Sam again in some way and that’s why Dean *has to* watch over Sam.

In this ten year time period, Sam has matured, changed, and gotten much harder. He doesn’t take bs, and he sure as hell isn’t giving his brother another chance to kick him in the teeth.

What happens next?

Plot bunny is for anyone to use, can’t wait to see what you do with it.