So let's say you name an amount that would make you happy, and you get it. Someone else names their (higher) amount, and they get it.
You're both happy!
Two months later you find out their amount.
Why does their happiness amount affect your happiness? Would you be happier if they were paid less? Maybe they should be paid the amount that would make you happy instead of what would make them happy? Why? Their circumstances are different, they are different people, whose requirement for happiness is different than yours.
Surely you'd be happy making more, as much as they do, but we already established that you were already happy two months ago.