Every survivor has a happiness value from 0 - 100. Happiness is affected by events that happen, and by the number of churches and bars in the fort. Low happiness leads to negative events like refusing to work, desertion, riots or suicide. The smiley face on the left side of the screen gives an average happiness of all the survivors in the fort.
After researching survivor management, you can see how a survivor's happiness is affected. All effects on happiness function through +/-happiness and are directly visible.
(Numbers below are written based on my memory and may be inaccurate)
These things increase happiness:
- Joining your fort (the initial happiness for all survivors, +25 for impossible and higher for easier difficulties)
- Having enough churches/bars in your fort.(+10? once in a while)
- Leaders bartending or preaching.(+15? once in a while)
- Policies such as private property, increased rations, luxury assigning, etc.(+ once in a while)
- Running water and electricity in your fort. (+? once in a while)
- Equipped with a child/pet.(+10/5 once in a while)
- Doing mission with friends.(+5? once in a while)
- Perks(Easygoing, friendly, musician, artist etc.)(mostly triggered as pop-up events)
- Using fireworks (+20-40? to one person)
- Having a good talk with the leader(the act of choosing a perk for survivors.)(+20)
- Taking time off.(+5 every day?)
These things decrease happiness:
- Starvation (VERY important, all other happiness issues are minor compared to this.)
- Playing to late game. (-10 once in a while)(Things like "This fort was better when it was small". Nothing can be done about this.)
- Not having enough churches/bars in your fort. (-5 once in a while)
- Policies such as sharing everything, decreased rations, luxury assigning, etc.
- Changing policies too often.(-20)(A warning will appear)
- No running water and electricity in your fort.(-10 once in a while)
- Boredom (-10? once in a while)(Can be prevented by recreation perk, which is granted by baseball bats, Andy's whip, etc.)(Speculation: happens when nothing effects the survivor's happiness for too long. Need confirmation.)
- Death of friends/pets/children
- Cannibalism of human/pets
- Being assigned to dangerous missions. (-10?)
- Doing mission with an enemy/nemesis.
- Not having enough spare houses in fort. (-5? once in a while)
- Losing a tile to attacks. (-20?)
- Losing Cultist/Devout perk because of having no church in fort. (-40)
- Perks(Downer, etc?)(mostly triggered as pop-up events)
Survivors with 0 happiness can often still work normally even under impossible difficulty. Several things should be treated with care though: Starvation will make survivors very unhappy; Rebellious survivor will refuse to work when too unhappy; survivors complaining to you in a pop-up event may leave your fort forever (practically dying) if not dealt with properly.