Relationships, good health, and experiences is where happiness lies.
Relationships can be lost by lack of money, good health requires the cash to buy healthy ingredients and you need money to go do stuff so money is the root of all happiness.
Imagine a billionaire with a terminally ill child. Having more money isn't going to bring happiness. I guess what I am trying to say is that no matter how much money you have, money alone will not make you happy. You need more in life.
Now try convincing me that he wouldn't be happier than a poor guy with a terminally ill child.
The rich person might actually be hit harder from it because they probably would have been able to try all sorts of expensive or experimental treatments, and then to have them all fail in the end and watch your child die would just drive home the point about there being so many things money cannot buy.
A poor person might be able to delude themselves and think "If I had more money, my child wouldn't have died." The rich person has a very different perspective. "Despite all my money, I was still powerless to stop my child from dying." It strips away your ability to delude yourself and reminds you that the universe is a cold place that does not care about you or your well-being.
deluding yourself to think more money would fix the problem as a poor person seems like it would make the person more bitter than anything.
Yes, I am not arguing that a poor person would be happy. I am merely providing examples of how rich people could be very unhappy despite being rich. ---