"I'm grateful for your feelings for me, but I don't feel the same..." Dick nervously smiles after you confess your feelings to him.

He didn't really see that coming from his best friend, he's both shocked and well he doesn't know the other feeling. Perhaps fear of losing his best friend.

Dick doesn't want to lose you just because he doesn't feel the same way, he cares about you but not in a romantic way. Is it the end of the best friendship he ever had? He really doesn't want that to happen.