Buffy is like a kind of awesome big sister figure. Not the one you grew up with, but that one that was too old to ever be annoying for you and now lives on her own and drives you around getting you milkshakes and taking you to movies. Like on TV. That's what Buffy is like for Sokka, only without the driving around. She's someone he can be himself around and say whatever it is that pops in his mind.