If that were true, then all humans and the people in all cultures would be equally unhappy. And everyone would be equally unhappy all the time. But they are not, partly because of people's expectation levels.
Happiness is an extreme, like sadness is another. You dip into those states for a bit, the homeostasis returns you to the middle. Now, we could discuss if anyone is content or not. The West sells happiness, the East tried to teach contentment. They are very different things.
Not true. Money does buy happiness up to a point. Going from sleeping in the streets to having a place to stay, food, security, and medical services makes a big happiness difference.
However once the basics are there, additional money makes less of a difference (but not 0).