Dean chokes the second the words leave Cas's mouth. He has been hiding by looking at the brands of the juices he agreed to bring for Sam (antioxidants, Dean. You need to check for the word on the box), making Cas deal with the lady that had walked closer after Cas chose to rub a stain of sauce from Dean's upper lip from one of the samples he received. It made it worse when Cas licked his thumb, before saying 'It tastes good' with a quirk of lips on his face that made Dean smile. And apparently, the action attracted one lady that was looking at their direction.
He decided to ignore her because that wasn't his thing. He loved Cas and all, but he wasn't able to look at the people in the eye while they thought they were a honey-loovey kind of couple and then went all feel-y with them. Cas's usual deadpan tone while talking was enough to make them go away, so he kind of choose to ignore the situation and let Cas take care of it. Though it seems that he still hasn't grasped about the social norms, because now he can feel the questioning eyes of the lady on the back of his head.
She chuckles nervously, as if expecting Cas to say that it was all a joke, but there's no reply from his part. Dean chooses to glance at them and sees that Cas is still looking at her without blinking, his head slightly tilted to the side. Dean honestly doesn't really know if Cas is doing it on purpose or not, but that moment is so Cas. One of those moments in which he realizes how different he is, how unique. That, for some reason, makes his heart flutter. Because this is Cas. This is the angel he had fallen for. Not just a bad-ass warrior or an ally that chose to fight with them, but also, his weird, dorky friend with a dry sense of humor.
A warm feeling slowly surrounds him and he can't help but grin so widely that he doesn't care if the lady is still looking at Cas with a weird expression. He presses a hand on Cas's shoulder, making him blink and turn slightly towards him, and then Dean leans and kisses his cheek. A loud smooch that even surprises the angel. His angel.
Dean's so glad that he managed to keep him.