Fallout Shelter Dweller Happiness [duplicate]
Aside from the post that Ben linked, dwellers often talk about the things they want, for example based on their S.P.E.C.I.A.L. they comment how happy they are with their current job(assigned room) and even how happy they feel if they're in the same room with one of their family members. So, listening to them could give you a little bit of idea about what to do.