So, I've started to notice something about Team Free Will, and Dean and Cas in general. Whenever they're standing in their epic trio pose, if Cas isn't in the middle, he's usually in the same place.

WHY.

IS.

CAS.

ALWAYS.

ON.

DEAN'S.

LEFT.

Headcannon that Cas always stands on Dean's left because that's the side his hand-print is on. He always touches Dean's left shoulder too. 

Cas likes to be close to the hand-print, and Dean doesn't mind.