Dean was not happy. Sam had turned him back sure, he was happy about being human again, but could he really stay human?
Listen, he wasn't human anymore. No matter how human he actually was, he would always be a demon on the inside. Like Cas, for example. No matter how human Cas became, he would always be an angel.
Dean was not happy and he knew exactly why. He missed it. He missed being a demon. He missed being able to heal instantly. He missed being confident in himself. He missed not being held down by his self-hatred. Dean needed that back.
He needed to be a demon. It wasn't his choice. He needed it like humans needed air. He needed it like angels needed grace. He needed it like demons needed chaos.
Crowley knew this. Cas knew this. Dean knew this. Sam didn't.
Dean was dying. He was dying just as Cas was dying. Cas's borrowed grace was killing him from within, Dean's humanity was doing the same.
Dean was sat in his room, he had barely left since he had been cured. Sam came in every now and then and gave Dean some greasy junk food or some beer, occasionally he got a salad or some water. Cas rarely came near Dean, when he did his eyes would flash blue and Dean would remember how his eyes were once black. Then he would be depressed for a while.