Sam and Dean have lost everyone they ever cared about. No one stayed around or they were killed somehow. They had no one....... but each other.
Sam feels distraught by the way he lusts after big brother. He doesn't know how to deal with the inappropriate erections he gets when Dean comes out of the shower or when he is all sweaty from the hunt.
Dean isn't oblivious to Sam's problems. He knows Sam has a thing for him, but he doesn't want to make a move in case he is wrong. So, he keeps on keepin' on, except he makes himself a little more appealing to the eye, or so he hopes.