A person might endure terrible suffering and not be able to express what is in his heart. There is no one to whom he can unburden his heart, so he remains deeply pained and worried. If you come to such a person with a happy face, you can literally give him life. (Rebbe Nachman, RNW #43)
Happiness is so important, it can actually bring a person to life. If you see someone who is so sad and completely dejected, and refuses to continue living because of the bitterness of suffering, when you bring happiness and make the person happy, you bring new life to that person.
This is the same as bringing a dead person back to life.
Adapted from Rebbe Nachman’s Soul, Vol. 2, BRI