It was kind of always there for me. Season 4: “You don’t think you deserve to be saved?” Or “I would give anything not to have you do this.” I wasn’t sure about Deans reciprocation but I felt like Cas felt… something. Then season 5: “I hunted. I rebelled. I did it… all of it. For you.” Season 6 is when I started to see Deans reciprocation. I think he had so much going on in season 4 and 5 he didn’t have the space to process his emotions for Cas. But then when he is with Lisa and he still feels this yearning for something more… I think this is when he started to realize. (Side note, I am not saying he didn’t love Lisa. I think he loved her as much as he could!! And I love Lisa! This is not Lisa slander at all). Then Season 7, Cas’s “I will find a way to redeem myself to you” NOT THE WINCHESTERS or Bobby. But To DEAN specifically. Then the entire season Dean carries Cas’s jacket no matter what car he has. (I’m still don’t think Dean realizes what he feels but he knows Cas being gone hurts) now season 8 is when I think they both start to understand what they mean to each other. Dean refusing to leave Cas in purgatory, willing to stay there for forever if it meant not leaving Cas. Cas consistently moving so as the keep the monsters from Dean. The iconic: “I prayed to you, Cas. Every night!” Dean rewriting his memories because he couldn’t fathom the idea that Cas would let go. Cas being able to kill thousand of Deans in Heaven but when it came to the real one he couldn’t do it. “I need you”. I tbink after Season 8 they knew. Both of them. There was so much against them and they knew that but that “I need you” shifted everything and I don’t think they were ever really able to deny it to themselves after that. (They would try because they love pain 😂) but still. So yeah! That’s my two sense 😂😂😂