Is depression defined by (expectedReality – Reality) >0 ?

No. That’s the equation for disappointment. But depression might be a good descriptor for a large difference between the two. Injustice and anger may also work.

Is happiness then described as (expectedReality – Reality) <0 ?

Maybe, but maybe it’s better called gratitude, or potentially, guilt.

Satisfaction is the state where expectedReality and Reality are equal.

For some expectedReality is fixed; car + 2.5 kids + job + golf.

For others expectedReality is a vector; today’s assets > yesterday’s assets, or Reality(today) + 1.

The difference between a fixed and changing expectedReality, is an expected(delta)Reality value also in the equation for happiness. It’s there that people can be better described.

If, based on life experience and wiring, tomorrow’s reality will be less than today’s reality (i.e., worsening health, closer to death, less money) then expectedReality is more likely to be fixed, and satisfaction achievable. If, however, there are no perceived limits on tomorrow’s reality (i.e., I could win the lottery, I could find a better wife, my kids will behave better), then expectedReality is variable, and satisfaction elusive.

This exercise is reductive enough, but my point is this: the perception of a better tomorrow may be the enemy of happiness.

Of course, reality cancels out somewhere along the way, too. But it’s an idea.