Well, it appears to be a scientific fact that it’s not true. No particular achievement or occurrence can guarantee long-term happiness by itself, because you will get used to your new status and your degree of happiness will eventually revert to something close to what it was before X, whatever it was, occurred.
8 Interestingly, Adam Smith, the intellectual father of modern economics, understood this point; he once wrote: “[T]he mind of every man, in a longer or shorter time, returns to its natural and usual state of tranquility.