>> Capitalism is built to make sure that psychopaths are never satisfied;
Although it happens all the time, it's not the nature of capitalism that makes it so, just the greedy potential of human beings. We each develop an inertia of life and capitalism (especially 2019's unfettered capitalism) definitely allows the rich to get richer, but that is not built into the system, just the majority of human beings. No, capitalism is also very good at letting those who earn a great deal of money share that money with those who have been less lucky in the roll of life's monetary dice. More simply put: systems designed/implemented by human beings are only as good as the human beings who manifest them. If they are callous, then the system will be callous, and that is precisely what we have today.
>> The fact that wealth and happiness are often mentioned together in articles and books is a very strong indicator that they are related.
No, that just indicates that most people don't understand the difference between pleasure and happiness. Pleasure is related to physical well-being -- i.e. Maslow's Hierarchy of Needs and the like -- and wealth is the easiest way to get any kind and amount of pleasure one desires. Happiness and/or its opposite is the accumulated results of our treatment of others; it results in a feeling that wells up inside of us. It is absolutely unrelated to any physical pleasure.
At the most raw, basic level of our universe are the physics-level automatic systems but at the most sublime level (and only for human beings) there is the Law of Karma. Because we are endowed with free will and are required to live in groups to survive and prosper, the universe is designed with an automatic feedback mechanism to nudge us toward selfless cooperation and away from the selfish competition of our animal physicality. As such, we quite literally reap the feelings we sow in others. That is why wealth has no direct bearing on our happiness, although it is very negatively correlated with the tyrannical corporate execs who have reduced their workers to 'human resources' to be brutally bargained with to maximize the owners' profit.
As surely as a person can be drunk on power or alcohol or drugs, one can be drunk on accruing wealth at the expense of others; in all instances, the hangover is unpleasant. If the intoxication leads to maltreatment of others, then that hangover will be a deep-seated unhappiness that can only be cured by creating fresh happiness in others; each day is a new day, i.e. "there's still time to change the road you're on". The problem is that the inertia of our lives combined with the general denial of the Law of Karma's existence usually leads the person to only dig the hole deeper.
The entire purpose of human life is to rise above our mammalian competitive instincts and self-evolve ourselves and our societies into cooperative groups of humanitarians where we are obsessed with compassion for everyone. That such compassionate regard for the poor is almost non-existent in today's world societies is the source of much misery and unhappiness -- misery for the poor and unhappiness for the selfish, unfulfilled wealthy who callously disregard their ability to uplift their fellow man.
There was a great story that popped up here (IIRC) about a successful health-care company whose owner was in the process of converting it to a non-profit. Because he had realized that his wealth was the direct result of his workers' efforts he out-of-the-blue gave large bonuses to ALL his employees (based upon seniority). That man gets it and his happiness should be held up as how capitalism is capable of enriching society.