Why are my dwellers unhappy? [duplicate]

Solution 1:

The key to happiness is to send dwellers do the job they are good at. If they have the same stat on multiple things (S,P,A) then you train them. If they have charisma you create a radio room and make them work there.

If nothing works, sending them to the living quarters (the house of love) should clear any dark thoughts! :)

Also some of them might want to explore, especially the ones with high endurance.

A good tip is to listen the people. If you zoom in the rooms, you will see them talking about stuff that bothers them or stuff they would prefer to do!

Solution 2:

If the dwellers have red on their health bar, they have radiation poisoning.

Give them a radaway, and they will be happy again.