People described as naturally upbeat are almost always doing something. They sleep. They walk a dog twice a day whether or not they feel like it. They have one friend they can tell the ugly version to, which is why the public version stays light.
Calling it a temperament hides all the maintenance, and lets everybody else assume they were issued the wrong equipment at birth.
Whatever brightness you carry, a good share of it is upkeep. Upkeep is something you did. It can be thanked the way you would thank a person.