Spock did it all the time as well, but he was Spock.
I think, in psychological terms, it's probably a sign of being comfortable with the other person and open to listening.
Maybe a bit of domination too, as is the case with putting your hands behind your head (which is basically telling the other person that you're so confident that you could break them like a twig if necessary, that you can present them with all your most vulnerable areas while practically handcuffing yourself).