Ok going back to Sam's Dreams maybe what he is worried is that Dean won't be strong enough to survive having a demon lover, that his mind, essential Deanness will be lost. Or that Dean won't be strong enough to pull Sam back to himself or at least integrate the two Sam halves

I also wanted to add that whatever your muse brings is great, but I am still hoping for Sam finding Dean after he attempts to kill himself. Yeah I know Sam will be angry but I guess I am hoping still for something more in his reaction and maybe get a little Sam point of view from Sam's side and not just filtered through Dean if we are lucky

Zaz