It's Mostly Their Dad's Fault

Supernatural started out as a horror show. The first season premiered when I was in high school, and I remember needing to leave the lights on while I watched; it was just too scary not to. But even in the first season, around all the terror, they started establishing the incredibly close bond of Sam and Dean Winchester. As the show progresses, we learn that the strong brotherly relationship is actually unhealthy codependency. Several times, the Boys have tried to part ways, and without fail are ultimately unsuccessful. Even at the end of season five, when Sammy was supposedly sent to hell and Dean was supposedly happy with Lisa and Ben, it took them a year but they ended up back together and Lisa and Ben were forced to literally forget Dean.

Neither of the Boys has ever had a functional relationship; the closest for Sammy was Jessica (who was killed by Azazel) and the closest for Dean was Lisa (though he was still pretty messed up and ultimately he had her and Ben's memories wiped for their protection). Jenna and I have discussed this and we realized that  they never saw a good relationship. When Dean was a toddler, John and Mary were having relationship issues, and then Mary died when Dean was four and Sammy was six months old. Once Mary died, John snapped and, though we know he had flings (thus we get Adam), he never had another relationship. Their secondary father figure, Bobby, killed his wife before the Boys spent so much time with him. The only long term relationship Sam and Dean know is their own, so it's no wonder they're dysfunctional.

Don't get me wrong, I love watching their complete devotion and loyalty to each other, even when everyone else is saying the opposite. It's one of the appeals of the show. But if they were real people, they would have serious psychological issues in connection to their relationship; the lengths they are willing to go for each other are unhealthy. So, yeah, they did some of the damage to themselves, but it's mostly their dad's fault. Because he could have let go of Mary, moved on, met a nice woman, and raised the Boys in a normal life.