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Looking back on it, Dean realized there had been warning signs - he'd just been too caught up in taking care of Sammy to notice until it was almost too late.
The first warning was when Cas would always excuse himself after dinner, locking himself in the bathroom for various.periods of time. Dean had just figured Cas was getting used to his new body. He had walked in on him jerking off once, and it wasn't something he really wanted to have a conversation about, so Dean figured he'd let him be. That was his first mistake.
The next warning came when Dean's razor went missing. He looked everywhere, questioning both Sam and Cas, but neither of them had had any clue where it had gone. That was his second mistake.
The final warning came when Cas had gotten wasted. Dean had thought it would be fun to get the former angel drunk, loosen him up a bit. But then Cas had started complaining about how hard humanity was, and how he was so tired, and he didn't know how to go on anymore. Dean had figured it was just a result of the vodka - it was a depressant, after all. Cas would be over it in the morning, he was sure of it. That was his last mistake.
Guilt-ridden, Dean sat in the chair at Castiel's bedside, wishing to go back, wishing he could've stopped Cas before it had come to this. He ran his fingers over Cas's arms, trailing over the scars that adorned his wrists.
"Damn it, Cas," he whispered squeezing the man's hand. He hoped he would wake up soon.