My father died of a heart attack at 36, due to a heart damaged by rheumatic fever. My mother did fine in the loving-care department, except in choosing my stepfather, who was abused as a child, an incipient alcoholic, and in turn, emotionally abusive to me. I too had my "nervous breakdown" at 44, due to the aftereffects of the abuse, the onset of type II diabetes, loss of my job, and the death of my primary emotional support (my mother and grandmother) a few years earlier. Modern psychiatry and drugs has helped me, but Nietzsche did not have those options, so I more than empathize: I sympathize with the great philosopher.
Nietzsche lived in a time when both human psychology and psychiatry were barely in their infancy. Sigmund Freud was thirty-two the year Nietzsche had his break-down: barely beginning his practice that would bring him both world-wide fame and notoriety. Carl Jung was thirteen, years from beginning his studies under Freud in Vienna, and many more years from asserting his independence of Freud and his former mentor's preoccupation with sex as the single driving force for humankind.
Besides a possible genetic predisposition to mental illness, I believe that it was Nietzsche's lack of loving, emotional support--unconditional or not--that was at the root of his breakdown.
As far as we can tell, Nietzsche never had a girlfriend--or a boyfriend for that matter--in his life. Even the beautiful Lou Salome seems to have been a friend, not a lover. He never had a mistress, although it was common for men in his position (university professor) to have a housekeeper or maid who did double duty. We know that he was socially awkward, thanks to his mother denying him attendance at most social functions as he grew up.
In The Prince, Niccolo Machiavelli stated that if a ruler could not make the people love him, he should make them fear him. I think that Nietzsche's "Will to Power" was derived from this idea. Nietzsche's near-total lack of love and affection in his life after his father's death led to this conclusion. This blinded him to the potential of any other motivating force for human happiness or success, in the same way that Freud's experience treating sexually abused adults--whom he could not believe were used in such a fashion by parents, relatives, siblings, and others in positions of responsibility--blinded him to anything other than the human libido as the primary human drive. Freud ignored and sublimated to the libido every possible alternative drive, including the drive for power. Nietzsche ignored every potential drive except the "Will to Power," including the Power of Love, both Agape and Freud's libido or Eros.
Nietzsche, like many individuals who have been denied--or have denied themselves--their share of love and emotional support in their lives, believed that self-interest was the basis for a system of morality. But as I have stated before, Thomas Jefferson destroyed the myth of self-interest as a system of morality in a letter to Thomas Law two hundred years ago [corrections and amplifications in brackets]:
"'Self-interest, or rather self-love, or egoism, has been more plausibly substituted as the basis of morality. But I consider our relations with others as constituting the boundaries of morality. With ourselves, we stand on the ground of identity, not of relation, which last, requiring two subjects, excludes self-love confined to a single one. To ourselves, in strict language, we can owe no duties, obligation requiring also two parties. Self-love, therefore, is no part of morality. Indeed, it is exactly its counterpart.' (The Complete Writings of Thomas Jefferson, Memorial Edition; Volume 14, page 140; 1904.) We can be responsible for ourselves; we cannot have a duty to ourselves; because duty involves not only our own desires, but in acquiescing to the wishes of others. [This is the hypocrisy inherent in Margaret Thatcher's statement, "There is no such thing as society: there are individual men and women, and there are families" (Woman's Own; London, October 31, 1987)]: [for even] the betrayal of the family, duty to the family must first exist; for treason [against the nation-state] to exist, duty must also exist. You cannot, per se, betray yourself; you can only violate whatever responsibility you believe you [may] have for yourself." (From "Libertarianism in Its Destructive Phase, Part 2," 4 September 2013.)
So we are left with a brilliant man trying to find meaning for his life in a place where none may be found. Power is a sterile and unsatisfactory substitute for having people in your life who care for you, who love you, and who want the best possible outcome for you in your life. Only those with narcissistic personality disorder, sociopaths, and psychopaths, because they lack empathy, compassion, and the ability to honestly relate with other human beings, will ever be satisfied with power as a substitute for love. And because the more we love, the more we can love, it--unlike power--is not a zero-sum game.