It's completely true, but it feels very untrue, which is why people doubt it any time the question gets asked. The reason they doubt it, is because they suffer in a world where money seems to solve most problems, which means it seems to alleviate most of one's suffering. What happens when your suffering is soothed temporarily? Well you probably have more room to be happy, right? Or you're happy just cause-- this problem, whatever it may be for you, actually seems to have gotten solved by this clearly powerful thing called money(What idiot would doubt the power of money?). Awesome, right? How is that not great and self-evident?
Except... people with lots of money, still suffer horribly. They commit suicide. And in many cases, any time they are happy, their money alone seems to often put their happiness into question if you look carefully. Sometimes you don't even need to look carefully. Conversely, people with no money can be blissfully happy, in more and less meaningful ways.
If you just want the answer stated directly, it's the next sentence:
You cannot be happy in a genuine way, without an ability to deeply understand others specifically in a way that has compassion for them. There is no genuinely happy person on earth, who is genuinely happy, and also totally self-interested, totally self-absorbed, and only interested in others superficially-- as means for their own comfort, pleasure, ends, and so on. It is impossible to ever be genuinely happy this way.
You should doubt this and test it as thoroughly and honestly as you possibly can, in a way that never closes the case, but continually re-evaluates and re-assesses, and adjusts ones certainty as slowly as possible.
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