Dean had successfully hidden what had happened to him for about two months, and by extension, kept his family safe from himself.
See, awhile ago they went on a werewolf hunt and well, you can probably guess how it went from there.
Honestly, the only reason he hadn't killed himself yet was because SOMEONE had to take care of Sam, and he couldn't do that if he was dead, so he decided to stick around and avoid the consequences.
Now though, he thought he might end up strangling his little brother.
"Sam, please just shut up for once! We have this argument every time, and every time we leave so just get in the freaking car!" Dean's voice had none of his usual sympathy for Sam. He was hoping that if they left fast enough, they just might be able to make it to the next motel before tomorrows full moon because he knew there was no way to convince dad to stay here that long, and he definitely did NOT want to change in the car with Sammy and his dad. He knew what would happen to them and he couldn't live with himself if he did that.
"Why are you taking dads side all of the sudden huh? Why can't we just stay for once?"
"Because people are dying Sam! Now just get in the car and stop being selfish!"
John Winchester was used to his sons arguing. I was a hazard that couldn't be avoided when you had kids so he thought nothing of it. That is until he heard snarling and turned around to see his youngest get tackled by a werewolf and a pile of shredded clothes where Dean used to be.