Words change meaning over time, and the way it happens can reveal secrets about the culture that’s changing them. Take ambivalence. For many of us, it has come to refer to an absence of opinion: “You choose, I don’t care.” Traditionally, however, it refers to the simultaneous presence of contradictory feelings and attitudes. It’s a surprisingly young word, coined by Swiss psychiatrist Eugen Bleuler in 1910. Soon after, it was adopted by Freud, who saw that ambivalence—particularly the coexistence of love and hatred toward the same person—was a fundamental aspect of human experience. You don’t need to read Freud to understand his thrust, you just need a sibling.

Melanie Klein, the great psychoanalyst and observer of children, had a more touching take on Freud’s theories...