What a nice gesture you made by giving the girl some bread. That gesture not only made the girl happy, it surely made you happy too. That's why I say that happiness is subjective and simple: perhaps what makes me happy doesn't make you happy. For example, in the case of the girl, eating made her very happy. When we have our food needs met, we don't see anything special about eating. But other people would be very happy if they could at least eat a piece of stale bread. Best regards.