Was there ever any doubt?

we didn't get to see dean reciprocate in 15x18 but at that point cas' feelings were the ones in question. not dean's. they trusted us to connect the dots and understand that dean's feelings had been communicated to us over and over and over for years. you can reasonably put a question mark next to dean's feelings up until 08x17 at the latest. but cas? cas' feelings could reasonably have the question mark all the way up to 15x18. yes, we can read between the lines on cas' feelings but there were legitimate questions that were unresolved. could angels feel love? could cas feel love? did cas feel love? and then in one fell swoop we got them all. yes, yes, and yes. cas was the x we were solving for. dean was the constant.

I am so convinced that if you sat someone who knows absolutely nothing about Supernatural down and made them watch a Destiel comp and told them "One of these guys is canonically in love with the other. Guess who it is." They would say Dean