Oh, people are welcome to write both! I am totally not trying to dictate how people should be writing Wincest here; I still read and enjoy fics in which Sam is the initiator. I'm just curious about why one is so much more popular than the other. And I think that this comment does help me understand; I agree that Dean needs Sam more - which might, of course, make him very hesitant to do anything that might damage the bond between them - and you're right about his obsession with protecting Sam.