People have a button that locks them in the moment the button was pressed for the rest of their lives.
Twist: People spend so much time worrying about not missing the right time to hit the button, they stop living.
Scenario: People who have already hit the button are effectively walking dead, locked in happiness, and whatever is done to them now won’t affect the time they have left. The bodies that go on are soulless and treated as such.
Second twist: Bodies are still conscious.