SASSY...except not

[hils]

So, I was just looking at this on tumblr and it got me thinking. Why is that moment there? What purpose does it serve? Is it just the writers fucking with us because the Cas fans have been demanding Cas get a hug since S4? Is it the writers demonstrating that Sam doesn't love Cas as much as Dean does? Or that Sam feels awkward hugging the angel he knows Dean is in love with?

I honestly don't understand. It has to be there for a reason, right? I'd just love to know the thoughts behind it.