When talking about happiness, psychologists distinguish between hedonic pleasure and eudaimonic pleasure. Hedonic pleasures might be what we understand by being 'happy' and eudaimonia might be what we understand by 'content' or 'satisfied'. One is charatcerised as pleasure (eating, sex, laughing) the other achievement (satisfaction of a job well done, overcoming difficulties to succeed, experiencing 'flow').
We often look down on hedonic pleasures, as though they are somehow less rarefied or worthy than eudaimonia. But psychologists have shown that the two are in fact interrelated. Eudaimonia is consistently accompanied by feelings of happiness. Enjoyment is routinely paired with eudaimonia. Indeed, 'flow' (the feeling of being completely absorbed in a task) is more likely to occur if one is in a happy mood. Happiness appears to heighten an individual’s capacity to discriminate between meaningful and meaningless experiences.
Why is this?
Nature rewards us for activities that are essential to survival. Those activities tend to be hedonic: eating, drinking, sex. Happiness in this form is often sneered at, perhaps because there are characterised as lower or vulgar pleasures.
But happiness is also linked to meaningful activities. Being absorbed in a task, trying, winning from a difficult situation, achieving something - these are all activities which have an evolutionary cost in terms of energy and time. Yet nature rewards us for doing them.
Happiness may play a role in enhancing the experience of meaning .... lending hedonic reinforcement to these eudaimonic endeavors
Professor Laura King, 2007.
In other words, nature rewards activities that it favours. By rewarding meaningful activities, it reinforces them. Truly, humans are unique in this respect. Nature rewards survival and procreation. But it also rewards effort and being exceptional.
To be truly human, we must strive to achieve.