If someone asked him, when exactly was it that the started to fall for Castiel, he wouldn't be able to tell you, for it was the littlest things that Dean loved Castiel for. The way how even when he'd told to get out of his personal space, the angel would continue to ignore it and get in Dean's space. And as much as Dean said he hated it he actually really loved it.
Especially when Castiel would touch him for too long than was appropriate for two friends. He didn't care.
He told all this to someone one night and do you know what their response was?
It was;
'I love you too Dean'.