He practically screamed with pain as he took Sam's form. He was getting all his memories and emotions. He didn't know how this guy got up in the morning. The second he was fully transformed, he saw Lucifer there taunting him. Since he wasn't nuts, he was able to banish him. He kept seeing Hell, though. It seemed even worse than Purgatory. There were only four inhabitants in the box, and in a way that was better than millions of monsters, but with two of them being angels it was almost even. He wouldn't want all that powerful rage concentrated on him.
The other leviathan was turning into Dean. He was practically drowning in guilt. What was this guy's problem? Who cares what you did to other people. He killed a monster so his brother didn't have to do it and he still felt guilty about it. He couldn't understand it. He felt like his job was killing monsters. He felt like protecting his brother was his job. He killed two birds with one stone. And he felt guilty. The really confusing part was that Sam was supposedly the crazy one.