I felt so angry and shocked when I first watched this, and just as angry (if less shocked) upon rewatch. Of course Dean's in pain and I cried when he cried. And I can understand how angry he is with himself as well as with Jack over what happened to Mary. I can see why he might be so upset that he would do something this drastic while telling himself and Sam that they were keeping Jack safe (from hurting people) by temporarily locking him in the box until they figured out a way to get his soul back. But forever? Buried alive forever??? Dean might be willing to sacrifice himself that way to save the world, but it's just not okay for him and Sam to do that to somebody they love. The fact that Jack got out is beside the point. The Winchesters never should have done that in the first place.