Sam is back from hell and nobody knows why. Mitch Pileggi was brought back from the dead, and nobody knows why either. Just a couple of miracles, cause that's how they roll in Supernatural.
So it turns out a year has passed, which puts season six in the future, but that's OK too. Dean has been in a very happy relationship with Lisa and her son for the past year, working as a car mechanic. Suddenly he is being attacked by genies (djinns) and Sammy boy is rescuing him from death.
It should have been a great moment for Dean, but when he finds out Sam has been back for a year, Dean is outraged. How could Sammy let him be in a positive and loving relationship with Lisa, for a year? Why didn't Sammy come back from hell running to be Dean's lapdog and punching bag? How could Sammy dare to have his own life?? So now Dean has to do the incredibly cliche & predictable break-up with Lisa, so he can be a "hunter" again.
Dean's attitude is beyond creepy and ridiculous. This series should have tried to keep Sam as the main hunter character working with the Campbells, and followed Dean and his family life as a subplot for a lot longer than just a couple of episodes. It would have been even better if Dean stayed in his "normal-guy" life, and got written off the show. It was better without Dean and his never-ending negative attitude and god-complex. He thought he was better than everyone else, but he was just an angry drunk in denial.