Ok, so I don’t mean this sarcastically, or in a subtext kind of way, I actually mean it in a I-believe-the-character-is-played-like-this kind of way. And here’s four things that prove it:
1) The profound bond thing:
It may not have been intentional at the beginning, and this line might have been dropped like it was no big deal (and probably also for comic effect), but this was one of the first signs something was up. Castiel running to them as soon as Dean calls while ignoring Sam for months is not only proof of their “more profound bond”, it’s also proof that nobody ever cares about Sam, but that’s a different issue…
2) Naomi Killing training thing:
You may have noticed that when Naomi was trying to brainwash Cas to make sure he would fight for the angels she made him practice killing Dean, and just him, not both the Winchesters or other humans. Weird, right? What possible explanation could that choice have? Fine, I can hear you protest that since they have the “more profound bond” she just wanted to make sure he would be ready to kill “his favorite” human, but I honestly don’t think that refutes my point, quite the contrary actually.
3) Give up everything thing:
For a brief time in season 10 – man, this show has been going on forever…- Castiel’s ambition was making up to what he had previously done and reuniting the angels, bringing everyone back to heaven. He had actually managed to get a whole army to back him up and everyone was ready to fight in his name. Well, as soon as Dean’s life is in danger it takes him less than a minute to give everything up just to make sure he won’t get hurt by anyone. If that’s not true love than I don’t know what is. Also, Hanna’s face when she realises Cas has picked Dean over her is the cherry on top.
4) Reaction to the brother thing:
when Dean calls Castiel “a brother”. He really looks hurt when Dean call him that, even though I can sort of explain this to myself by exercising a little doublethink, which is basically a shipper’s most powerful tool. This moment more than ever has convinced me that although Destiel may never become canon, Castiel is definitely canonally (that’s not a word, is it?) in love with Dean.
Bonus thing: every other time Cas is just staring at Dean for no apparent reason.