As lovely as hugs and peach fuzz and I need you were, for me, this was by far and away the most romantic moment of the episode.
Nothing screams true love like saying to someone, “Together you and I took on your greatest fear and defeated it, and I have unwavering faith in us that we could do it again”.
I also feel that this moment was one of the most powerful ones. I loved the “I need you”, but I think I loved this part even more. This is Dean saying: “Bring it on. Who cares, I have us.” It doesn’t matter if you see that as platonic or romantic (after everything that’s happened - and be established by the writers, I can think of no scenario in which their relationship doesn’t have a romantic side to it, however unconventional or non-traditional it might be, but still, to each his own). Whatever kind of love you read it as, I agree with Flutie, this is true love.
Dean has abandonment issues. He’s been taught all his life that if you get close to someone, you’re certainly gonna get burnt. That you’ll better never get too attached. (Better: he’s been taught this by the hunter life, and ultimately by his father.) And it’s no use pretending that’s not true. It doesn’t have to be like that, but there’s always a chance of being left or betrayed or both, people make mistakes and the more you love someone, the less chances you have of not getting broken at least once.
One of Dean’s biggest fears is being abandoned, but one step at a time he let Cas in anyway. Cas was the only person that ever loved him for the man he is and Dean let him in.
And then Cas did betray him, his worst nightmare did come true, and it almost destroyed him. Just look at season 7. Cas showed up again… and Dean chose to fix it. For him, what happened with Cas could have been proof that indeed you will always suffer if you get close to someone, but the point is that he stopped focusing on that. He didn’t wonder whether it meant proof or not, he said “fuck everything, I don’t even care, I only care about making this right” because it doesn’t matter if it’s true or not, and it didn’t matter to him if he got burnt, the point is that for him, it is worth it.
So Dean trusted Cas again despite knowing the risks and having lived the pain of betrayal and abandoment, because it was still worth it. He’s spent a year in Purgatory and he would have stayed there ad libitum, unless he can leave with Cas, because it would have been worth it. And he’s ready to be chased and have to fight when they get back on Earth, because what he and Cas have is worth all of that.
It makes sense: peace and safety and lack of pain don’t mean much if the person you love is not there to live them with you. Dean would chose all the danger and sorrow over that tranquility anyday, if it meant having Cas there with him. He’s stated that with actions and words, and you don’t get much clearer than that on how you feel about someone.