He’d had issues with his temper his whole life, why should now be any different?

Leave it to Dean to attack Ruby when all she was doing was making him stronger, making him better. “I’ve been out hunting demons, Dean! Finding out where Lilith is and what seal she’s going to go for next. Cas has got nothing and if he does, he’s not sharing. We’re finishing this job. You don’t like what I can do, don’t trust it but it’s a part of me, Dean. I can do something good with it. Find Lilith and put an end to this.”

Because he had to prove that his power could be for something good. That he wasn’t some monster that their Dad thought he’d be.

“I’m doing the job.”