all the fic I wanted to write but didn't

Sam is the aggressor, the instigator, in their physical relationship.

Dean essentially raised Sam. Other than that, their childhoods were fucked up.

Sam subconsciously feels entitled to Dean in every way. Dean could never say no to him, so there are no limits to what Sam feels comfortable asking of him.

And it started a long long time ago.

And it continues, throughout. Sam is Dean's baby brother. Because of that, he's never started anything, but he simply cannot say no to Sam.