Procreate and Die
Nature doesn’t require that you be happy or sane; it just requires that you procreate and then die.
Happiness and misery are no concern of nature. The machinery of nature wants you to procreate, live long enough to rear your offspring, and then fade into oblivion. During this journey, life has no regard for your mental state provided you get the job done, and so it becomes your responsibility to ensure, as far as you can, that life is tolerable and maybe even happy. Our natural inclinations would tend to push us in the direction of misery, as we are burdened with obligations and stressful situations, all of which drain our energy and often leave us anxious. This being the case, it isn’t too difficult to see that happiness is not a natural state and requires an individual to have self-knowledge and knowledge of the world. It needs us to be unusually self-focused and not to accept the default state of being a bag of genetics that nature can use to create another generation, which in turn will do the same. Happiness is your responsibility. Take the hand that nature deals, and happiness may be elusive. Take the trouble to come to understand yourself and the way the world works, and happiness (or at least less misery) is a genuine possibility. Nature doesn’t even require that you be sane; pass on your genetics to the next generation, and you might as well have been Einstein as far as nature is concerned. Happiness is against nature.