This doesn't sound true or only insofar that it makes you unhappy if you do a comparable amount of work but don't receive a comparable compensation. So I would say that not receiving your fair share makes you unhappy but not only having less then others.
People still experience diminished happiness if they feel their relatively low quality of life or social status is because of their perceived own shortcomings. This is often how depressed people feel.
Depressed people feel depressed because of a chemical imbalance in the brain, not because of the ready they think. Having a chemical imbalance allows all sorts of thoughts, but they should be considered in context of someone who may already be incapable of feeling happy for other reasons.
