Just plain stupid...
For those who haven't read this article yet go on and read it first:
A Very Special Message From Supernatural Showrunner Eric Kripke
I feel like we should apologize to Kripke for even making him clarify that in the first place.
The writers did not need to emphasize of the cryptic message given by Lillith.
It was blatantly obvious that Dean being a dick was not the reason why he was infected.
Sam clarified that the first time around when he said that the sickness spread to those who use fear as a weapon. And if they remembered the scene at the morgue earlier where Dean bullied that guy into performing an autopsy.
As for Sam. He was immune to that demon virus in 'Croatoan', this is just an infection.
The writers have never given us an easy clue before, so i don't expect them to start now.