I think it's just a convenient way of getting round the 'abuse' angle, seeing as Dean was the one who petty much raised Sam (the question of whether Dean was 'raised' at all is...contentious). If you're going for the non-angst angle then Sam being the initiator is pretty much the only way you could do that without veering too wildly out of character.

Oh, Croatoan! Dean's whole "I'm tired, Sammy" speech just, NNGH, MY HEART.