And I think the fact that Pharm is related to Korn is what really shook Dean at that moment. He had been looking at the whole situation they were in and recognizing Korn in himself. He was always mad at Korn for doing what he did, for killing himself and causing In to do the same. Maybe he turned all his anger towards Korn and it was easy to blame Korn because Dean is Korn.

But then he found out that he’s related to In and just got angrier at Korn.

And now he found out that Pharm is related to Korn... and he has the flash backs of In and Korn being together... Maybe Dean thinks about the ways in which he is like In as well as Korn.

Then thinks about the way Pharm is like Korn as well as In.

They share each other’s traits because while Korn was reincarnated as Dean, Dean is related to In. And that means that In was reincarnated as Pharm, Pharm is related to Korn.

We, as fans, have known this for a while now. But Dean is just figuring this all out right now. And now he’s suddenly so scared that something might happen and while Dean would never repeat Korn’s mistakes, he isn’t absolutely 100% sure that Pharm won’t repeat Korn’s mistakes now. 

So he has his moment of absolute terror at the idea of it. That his sweet, soft, beloved Pharm might ever be so hurt as to do what his uncle Korn had done. 

Then Dean turns into his great Uncle In, and begs his boyfriend Pharm not to leave him. To think of him. To not do what Korn did to In, not ever. To not leave him.