I actually hate this moment. It’s Cas thinking he has to look perfect, put on his armour of the right clothes and a smiling face, for Dean to accept him.
He thinks Dean was looking at him weird before not out of worry, but out of rejection.
He thinks if he doesn’t provide the right image and attitude, Dean will send him away.
[if you think about all the times Dean makes fun of him or treats him rough, he has reason to].
This moment broke my heart for Cas. If Dean hadn’t tried so hard not to look affected and happy to see Cas clean and safe and gone and hugged him then, maybe it would have been a little less heartbreaking.