Pretty complicated subject but I think some of it is basics and some of it has to do with the world we live in.
In western and 1st world economies money is very important so I think it has alot to do with that.
I remember seeing a video about "the happiest people on earth" and it was a small country where the king actively tried to promote
a HDP instead of a GDP(happiness domestic product) and the people were very happy. Also islanders are very happy and they are not rich so
again I think it is partially do to the way life is setup in 1st world economies. Gorillas playing in water are very happy but they don't have money.
So security is important to happiness, health, and relationships which money affords in 1st world economies. I often see rich people excercising much more than others. Stress produces depression which can happen if you don't have money. It might also have to do with genetics - let's face it some people are just smarter than others and this makes life much easier for them when there is alot less stress to be able to achieve something. But happiness doesn't just come from having money alone. If there was no such thing as money but you could have all the things money affords would people be happier? I think so.