The only thing I can think of is maybe, after 40 "doggy" years in hell, he didn't just get tortured, maybe he did some torturing himself?

I have no theory (it must be bunnies! Sorry, couldn't resist), but I think it's an older secret than that... because remember Bloody Mary? Dean's eyes started to bleed, and the Kripkeeper said that we would eventually find out his secret but we never did. Maybe it's the same secret.

Personally I think that Kripke really quickly just said that that secret was the reason that Dean got the ghostsickness, because all the Deangirls were going berserk over the implication that Dean's a dick.

I thought him getting the ghost sickness was a weak plot point, because although he uses intimidation tactics- so does Sam. If they were going to make Sam immune, it should have been because of his demon blood. Overall, I just thought the plot was weak in that episode. And scaring the ghost to death? Kinda lame. The writing really could have been tightened up quite a bit.

I think that something happened to him in hell. =( <3 something that he remembers. my poor Dean! ='( lol <3

@fanfly:
I LOVED the scaring the ghost to death. Yeah, it was a bit weird, but this was different from other ghosts they've seen. This one had a really moving backstory, other than just wanting reeeeveeeeeenge, so I liked that they had to use more than the old salt 'n burn to get rid of this unusual ghost. Besides, I liked that even though it was a ghost, scaring him by reenacting his death was really cruel and sick and made you feel sorry for something that was killing other people off.

Oh, and Sam uses intimidation tactics too, but he's generally the nicer brother.

Exactly! It made me sick to see them do that to that poor guy. And they were pretty flippant afterward. And I really don't think the ghost was purposely making people sick. It seemed accidental to me- or at least he only meant to infect the guy who killed him. And that brings me to another weak point. If ghost sickness can be passed through a handshake or sneeze, then everyone would have it! Or at least a lot more people. Pretty lame IMO.
And about Sam, generally he acts the nicer brother, but did you see him with that demon in Metamorphosis? And the Crossroads Demon? I really don't think he's any nicer than Dean, he just acts less overtly obnoxious.

I don't think it's a dark secret. I think he just finally remembers what happens in hell.

i'm really not sure
some r saying that it could be he was tortureing other souls in hell but i have a hard time believeing that because well they r already kind of makeing sam a little darker this season and i don't think they would do that 2 dean as well. and if u think about it would an angel really save dean if he would have done something like that.

I don't think that sam is any nicer than dean is but i think it may have something 2 do with why dean's eyes bleed when he was confronted with bloody mary of course then he could have felt guilty for something he had no control over and that would be just like dean but maybe it was more than that.....i have so many other thoughts about this but i think i wrote enough :)

I was thinking of that too. Why were dean's eyes bleeding in bloody mary? what I think it could be is that in In The Beginning he tells YED all hes got to do thinking its his grandfather, he tells him his got to kill her mother to get to his brother.
I cant picture him torturing someone because to start getting evil in hell werent you suposed to be spending more than 40 years? thats what I think I dont know :D

I don't know, 40 years is a long time. After being tortured, it would be natural for Dean to lash out at someone else. I have a hard time believing he'd torture a freshly fallen soul, but a demon? He'd go to town on a demon! And can the angels see into hell? Do they know everything that happened to Dean down there? Plus, I don't think they chose Dean because of his, ahem, untarnished soul ;D. They chose him because he's the only one who can get through to Sam and steer him down the right path.
And about the Bloody Mary thing; like fifthbitten said- Dean could have been involved in any number of situations where someone got hurt because of him. What pops to mind is the incident revealed in Something Wicked. When they were little Sammy was hurt by that Shtriga because Dean left him alone AND other kids were hurt because he ruined the hunt and the Shtriga got away. So I don't think that has anything to do with this. I think this is a new secret.
Anyway, can't wait till 4x10!

I'm thinking it has something to do with the Bloody Mary thing, but that also could have been unrelated.

Yeah i think it was the fact that Dean remembered what happened to him in Hell and he Didn't tell anyone... (mostly Sam) and maybe Dean did some bad things in Hell... just to survive...