Season six had a daring structure: without telling the audience why, Sam changed substantially, which in turn changed Sam’s and Dean’s dynamic. The entire enterprise had a noirish feel as the writers decided to show the debate over what exactly makes us human without setting the scene first. It was a risky move, because many viewers were very uncomfortable with the way they viewed this new Sam and Soulless Sam stuck around for half a season.
I found the arc very satisfying, as part of Sam’s closed off nature has always been his fear of what exactly he is. What does it mean that he has had demon blood since he was a baby? What is the impact of having his life shaped by demons? Of being bred to be Lucifer’s vessel? I think Gamble was right on the money to have Sam realise his soul is the most important part of him and whatever else he may be, he has the crucial element to being human. And it was a typical Supernatural move to have Dean have to decide to return Sam’s soul at the possible expense of his life. There is always a cost for these two.
Dean, of course, also had his own changes to deal with in season six. His journey was to try and retire from hunting to have a family of his own. Again, some viewers were not comfortable with a Dean who had a different focus, doomed as his efforts inevitably were. But to me, this story line had to be done, because Dean’s longing for his own family had been shown early in the series. It’s too important a character point to remain unexplored.
John Winchester laid two conflicting charges on his young son: be the perfect caregiver for Sam and be the perfect hunter for his dad. John himself could not handle both parenting and hunting at the same time, but this is what he expected of Dean. Dean wears his hunting face openly and that is the aspect by which most other characters define him. But those who know him best realize how important it is to Dean to be able to love.
Dean’s desire to keep Sam safe and his birth family together is the most obvious sign. There are others. John Winchester tells his sons he wishes he could help Dean have a family of his own. The Djinn’s poison gave us another peek into Dean’s hidden dreams, as did the dream walking in "Dream A Little Dream of Me." In both cases, Dean wanted an ordinary life with a kind pretty girl next door, eventually embodied by Lisa Braedon.