Was just thinking about this and maybe someone can develop a story with this plotline.
MC is cursed - with always being happy.
How can being happy be a curse?
Possibilities:
-may have severe chronic pain that no one believes exists because he is always happy
-may end up estranged from family and friends due to his always being happy even in extremely sad circumstances (death of family member maybe)
-may end up imprisoned and convicted of a murder he did not commit because no one believed him due to his happiness even under questioning about the murder
Those are just 3 possibilities that I came up with...