As others have said, money gets you stuff, and certain stuff will get you happiness, so using the transitive property of equality, money=happiness. That isn't the only way to get happiness, though. Happiness is subjective, someone that's poor and is constantly working will be extremely happy to get a few days off. Whereas someone that's middleclass-rich will be far less happy from just a few days off and might require an entire week off to have the same happiness as the guy that is constantly working.