That's so you, the player, can select your 'disposition' how you'll respond to people. More specific interaction with NPCs will be done via a radial menu which will hopefully, but not definitely, be in the update. Your character will generally respond to NPC barks automatically, and this specifies how you respond to them.
And this is how we tie in the negative mood moodles with real game effects. If your character is angry or unhappy, it may lock off the happy, or even the neutral ones, and your character will be forced to reply angrily to any NPC. You'll be in the situation where you'll be avoiding your teammates, worried about pissing off all your fellow survivors when you're in a ****ty mood, or getting on the wrong side of an aggressive shotgun toting NPC while wandering, which will give the player a reason to try and improve it and maintain it.