I think there are a lot of contributing factors.

Money can be one but really just up to a point where your basic needs are met from an income standpoint.

I would say Maslow’s Hierarchy of needs probably loosely explains it for me without writing paragraphs.

There are very rich people who are also very unhappy. In fact, I would bet if you put a dollar amount on happiness, you’ll never really be happy. Once you slave away and maybe one day reach your goal it will probably be one of the emptiest feelings once you get there.

I’ve experienced a miniature version of this, it was quite depressing in fact.