What indicates a good fit with the nature of an organism? One sign is the continuation of the species. This may involve high birth rates — as Will notes — but not necessarily so. Another sign is physical thriving as apparent in rising longevity and increasing body size. In organisms with the ability to makes choices there is still another marker: how well they feel. Affect typically informs choosing organisms about whether they are in the right pond or not; they tend to feel good in habitats that fit their repertoire of adaptations and tend to feel bad in environments that do not. In higher animals, affective experience also hints at the appropriateness of some behaviors, such as mate selection. This affective orientation system exists in all mammals and serves to ensure the gratification of “needs” of which the organism is typically unaware. Like every system, it is not flawless.
In humans, evolution has added a cognitive module to this affective orientation system, but has not replaced affect with cognition. Our affective experience stills guides us to gratify our needs, and as a result we tend to feel poorly if we fail to meet them even if that fits our ideals. Affective experience also dominates our evaluations and, in particular, the evaluation of life as a whole, that is, happiness. As such, happiness also reflects need-gratification.
Will rightly observes that nature has no incentive to keep us happy. Yet nature has an incentive to guide us to the right ponds and to keep us fit. Nature seems to use happiness for that purpose. Negative affect pushes us away from unlivable environments and positive affects pulls us toward behavior that keeps our body and brain in good shape. In this sense, happiness is also indicative of human thriving.
Human nature evolved in the conditions of hunter-gatherer existence. Present day conditions are much different, but can nevertheless fit that human nature quite well. If we follow Maslow in his distinction between “deficiency needs” and “growth needs,” it is fairly obvious that present day Western society meets deficiency needs better than ever before. Less obvious, but still plausible, is idea that this kind of society also caters to human “growth needs” quite well. There is a lot of challenge and variety in our society, not only in market competition and in sports, but also in stimulating discussions such as this one.