This conversation between Sam and Dean took place in 10x10:
Sam: You know what Cas said about needing a powerful force?
Dean Yeah, so?
Sam: So, I’ve thinking. Look, Cain still has the Mark, right? And he’s lived with it. For years, he’s lived with it. So yeah, the Mark is strong, but Dean maybe there’s a part of you that wants to give in to it, and maybe you have to fight that, you know? Maybe part of that powerful force has to be you.
It’s very interesting that Sam is the one to remind Dean (and the audience of the show in case they’d forgotten) that Cain still has the Mark and has lived with it for years without being a stone cold killer.
However, the powerful force that helped Cain stop killing was LOVE. The love HE felt for his wife Colette due to her acceptance of his past actions and her faith in his being better than that. Cain’s “cure” was never Colette. The powerful force was the love CAIN FELT for her because even after she was gone, he didn’t become a murderer again. HE was strong enough because of her, thanks to her, because he didn’t want to disappoint her. Nobody gave him something magical to stop. The power was within him. HE was strong enough to do it. BUT he wouldn’t have found that power if it hadn’t been for his feelings for her.
If you search TV tropes, you’ll find a very common one called “The Power of Love”. Do you know what is the first thing you can read when you click here? "Love is a very powerful force." It caught my attention that in 10x10, the phrase “poweful force” was said 3 times (the first time by Cas and then the two above by Sam).
"Maybe part of that poweful force has to be [Dean]"? Is Dean as strong as Cain? I wonder… What did Cain have that Dean doesn’t?
If only Dean had a Colette in his life *cough cough*