Lots of inspiration here.
I just read The Happiness Hypothesis by Jonathan Haidt. A great read. I recommend it.
Human beings get more out of striving than arriving. It's part of our genetic programming. That's why we love losing weight so much, if we are overweight, of course, and in some cases, unfortunately, even if we're not. And why we get so miserable when we stall, or gain, even if we've lost a lot. But to really lose weight we need to retrain ourselves so eating the way we need to eat becomes a habit. That's not exciting and we are rather programmed to want excitement.
Another part of the human psyche is to compare ourselves with others. If we think someone else is better off than we are it makes us unhappy, all other things being equal. If we look at ourselves through the eyes of society and find ourselves wanting, we are unhappy.
Haidt says that our expectations adjust, so if we become disabled or sick, we adjust our expectations downwards and any small win or satisfaction makes us happy. In a way, the better off we are, the more it takes to make us happy.
He also says that if we have good, caring relationships with a few people, we are more likely to be happy than if we don't. Being fat doesn't stop that, far from it.
How we feel physically does make a difference to quality of life, though, in my experience. I feel better now I can more more easily, digest better, etc. I am less sensitive to heat and able to stand up without feeling faint. All that may not make me happy but it makes me enjoy life more.
Just my two penn'orth.