I think the secret of happiness is that you’re not supposed to be happy.
Not for for very long at least.
Rather than striving for happiness, I think we should be striving for contentedness. I think happiness may be a byproduct that cannot be created directly, but can only be cultivated indirectly by developing contentedness.
The human mind (and likely more influential, our emotions) seem designed to generate dissatisfaction, apathy, taking things for granted, and generally being unappreciative of what you have, and as quickly as possible.
I don’t think we can change this. And I think trying to change this is what leads to a lot of unnecessary unhappiness. But I also think we can be aware that this is the way the mind-emotion system functions, and plan accordingly.