Yeah, Sam is clearly too empathetic to be Sheldon-like, and he's gotten good at appearing to fit in instead of choosing to stand out. His social skills are much more in evidence than Dean's in certain situations. (And it's not like I don't think Dean knows how to behave most of the time - after all, Sam had to learn social skills from somewhere - so much as he just chooses not to as part of his own self-image).