"Everybody in the world is seeking happiness - and there is one sure way to find it. That is by controlling your thoughts. Happiness doesn't depend on outward conditions. It depends on inner conditions.
It isn't what you have or who you are or where you are or what you are doing that makes you happy or unhappy. It is what you think about it. For example, two people may be in the same place, doing the same thing; both may have an equal amount of money and prestige - and yet one may be miserable and the other happy. Why? Because of a different mental attitude. I have seen just as many happy faces among the poor peasants toiling with their primitive tools in the devastating heat of the tropics as I have seen in air-conditioned offices in New York, Chicago or Los Angeles."
- Dale Carnegie, How to Win Friends and Influence People