For the essence of love is kindness.

Robert Louis Stevenson (1850–1894) Scottish essayist, novelist, poet

Essay (1877-02), “On Falling in Love,” Cornhill Magazine, Vol. 35
    (Source)

Collected as "Virginibus Puerisque, Part 3" in Virginibus Puerisque and Other Papers, ch. 1 (1881)