Funny that it took Dean until he was sitting on the forest floor in none other than Purgatory after killing a particularly nasty group of leviathans to realize that he had fucked up. He isn't headstrong enough to let it escape him that the fight would have been a lot smoother if he had had the help of either Sam or Cas; the only hunters he can put his faith in.
Of course, Sam was back in the bunker, hopefully safe and sound, making up for lost time with Eileen. It was this line of thinking that sparked a dangerous, but persistent, thought in Dean's mind. Sam is not the only one who should be making up for lost time. The difference is Sam had the excuse of Eileen being dead. It's a bit hard to pursue relationships across dimensions.
Cas, on the other hand, has consistently been right within reach. The issue has always been Dean being too afraid to put his hand out. Of course, he's been close to it before but always pulls back at the last second. Examples being discarded notes, hidden mixtapes, whispers lost in the wind.
Dean realizes with a jolt of self awareness that he was in Purgatory when he first allowed himself to consider the strength of his feelings for the angel. Similar to his current situation, they were not exactly on good terms when they entered the woodsy hellscape the first time around. This time, Dean knows that it's almost entirely on him. He mentally kicks himself as he remembers the harsh words he has thrown out every time Cas attempted to initiate genuine conversation. Cas, the sweetest, most patient and loving person Dean has ever met. Cas, the fiercely protective warrior. Cas, the falling angel who has lost and sacrificed just as much as Dean has, if not more.
It takes Dean a while to be able to understand the reasoning behind his own actions. It isn't as if he ever wanted to hurt Cas. The issue with Cas being incredibly patient is it's rare that he stands up for himself and walks away when people treat him badly; especially the ones he loves. At some moments, the once confused and stoic angel displays far more humanity than any hunter Dean has ever met. Typically, it takes humans many years to learn their own worth and acknowledge that sometimes the ones they hold closest can be doing the most damage. This was never Dean's intention, but he accepts that he had been lashing out in a subconscious effort to ease the pain and stress of Mary's death and the end of the world. By the end of this impromptu self evaluation session, Dean has come to the conclusion that they have been processing the same earth shattering events, but separately. Now, there seems to be a better way to go about this. It isn't an unknown fact that Dean and Cas are just better together.
As there is certainly no cell reception in the land of monsters, Dean decides that the best way to reach and then find Cas is to pray to him. Now that he's mentally able to take a better look at the whole picture, he realizes there is nothing he wants more than to run right into Cas's arms and tell him that he's sorry and that he loves him and that he knows everything is a disaster right now but that now he believes they can get through it if they stick together. There really must be something purifying in the cold, Purgatory air.
"Well, here goes nothing," Dean mumbles.
"Hey… uh… Cas? Can you hear me? I know you could hear me all those years ago when we were here. Funny, I never really thought we'd be in this situation again. You'd think I would be prepared for anything at this point. But the truth is, Cas, I'm just making it up as I go. I try to keep Sam convinced that I can think of a solution to any problem that arises. It's been that way since we were kids. Hell, John raised me to be harsh and to the point. There wasn't much room for emotions along the way. I now know that's not the type of person I wanna be. Cas I'm so sorry. I never should've taken my mom's death out on you. It wasn't your fault. We all saw the problem in Jack. Hell, you've been dealing with losing Jack and I've been making it all so much worse. You've always treated me with so much patience and care when I don't deserve it at all and you don't know how much I appreciate it. I wanna be honest with you, I'm not sure how we're gonna get out of this mess that Chuck created. It might really be the end of the line this time and that terrifies me. The one thing I do know is that I'm gonna go down swinging and I want you and Sam there with me. It has to be all of us, no matter what happens. I didn't mean it when I said you're the problem. That couldn't be further from the truth. Cas, I don't know what I'd do without you. Dammit, I've been dancing around this for ages; I love you, Cas. I love how you're always there even through all the crap Chuck throws at us. No one other than Sam has stuck around me for so long, and he has to, he's my brother. Even after everything I've said and done you still choose to stay. You help me find some amount of joy in my life which as you know can be so hard. I'm so happy that I've been able to watch your transition from a badass, but uncaring soldier to someone so thoughtful and warm. I'm so proud of you, Cas. I wish I knew how to say all this to your face because you deserve to know that you're loved so so much."
At this point, Dean has his face buried in his hands in an effort to hide from the strong emotions and the weight of what he had just confessed. He stays this way for about 30 seconds before he feels a strong hand grasp his shoulder. Dean raises his head to meet swimming blue eyes. He quickly jumps to his feet in surprise. All Cas manages to get out is "me too", before getting swept up into a hug that neither of them wants to break free from.
Cas says, "I know we have a lot to work through between us as well as the apocalyptic crisis we've got on our hands. But right now I just need you to know that I forgive you and I love you too. I think I've loved you since I first laid eyes on your soul down in hell. I certainly didn't understand it then, but-"
Dean cuts him off by pulling him into a long overdue kiss that seems to stop the ticking clock.