You aren’t the only one to ask me this, pagestiel, so I thought I’d answer your ask publicly. Hope you don’t mind.
As to the specific moment when Dean started to fall in love, I’m not sure you can really pinpoint it like that. I don’t know when I fell in love with my husband, for instance, or how, or why. Love is something that happens by degrees, over time, when we aren’t watching it; and it keeps happening day after day, over and over, long after we first acknowledge it.
I do know when I first realized I was in love with my husband, however, and likewise I think we can pinpoint the exact moment Dean first realizes that he’s in love with Cas — not just that he’s attracted to him, not they’re just really awesome pals, but that there’s something different about this relationship, and he’s actually in fact head-over-Timberlands for the guy.
In “The Third Man”, when Dean prays to Cas, the angel shows up in the hotel room almost immediately. This is the first time Dean has seen Cas for a year. Dean’s reaction to Cas’s appearance is easily overlooked, because of what follows next (the “profound bond” line), but when he first spots Cas over Sam’s shoulder, he does a visible double-take; and when he meets Cas’s eyes, this is what he looks like:
I wish I knew how to GIF, because in the scene you can see that Dean stops moving completely. His open hand goes slack. The blood drains from his face. For a moment, he even visibly stops breathing. He is, for lack of a better word, thunderstruck.
The camera cuts back to Sam for a reaction shot, and then back to Dean for a close-up, where we see:
Dean’s eyes have widened considerably. His mouth has fallen open. The muscles in his jaw and around his eyes have gone completely slack. He still does not appear to be breathing.
Dean has — and very obviously so — missed Cas more than he ever realized.
This, I think, is the precise moment Dean puts two and two together (not incidentally, Cas basically says what Dean is thinking about the “more profound bond”). and figures out that whatever piece is missing from this thing he has with Lisa, Cas has had it, and he’s had it all along.
ETA: destiny-and-deerstalkers was kind enough to GIF this part of the episode for us. Go check it out; the GIFs capture what Dean’s doing far better than my words do. Thanks D&D! :)