The answer comes from the realization that genes for behavior, as well as for malaria resistance and teeth, are subject to natural selection. Why Is Sex Fun?: The Evolution of Human Sexuality

“I wanted to excel in your eyes, show my strength of will, wait before writing you, first finish a document, but the room is empty, no one is minding me – it’s as if someone said: leave him alone, can’t you see how engrossed he is in his own affairs, it’s as if he had a fist in his mouth. So I only wrote half a page and am once again with you, lying on this letter like I lay next to you back then in the forest.” (16 July 1920) Letters To Milena

To demand of strength that it should not express itself as strength, that it should not be a desire to overcome, a desire to throw down, a desire to become master, a thirst for enemies and resistances and triumphs, is just as absurd as to demand of weakness that it should express itself as strength. Genealogy of Morals

One's emotional state is always determined by the oddest and most accidental things, and it is precisely the most superficial factors that often fortify or diminish our courage. Beware of Pity