Perhaps I know best why man alone laughs: he alone suffers so deeply that he had to invent laughter.

[Vielleicht weiss ich am besten, warum der Mensch allein lacht: er allein leidet so tief, dass er das Lachen erfinden musste.]

Friedrich Nietzsche (1844-1900) German philosopher and poet

The Will to Power [Der Wille zur Macht], Book 1, Part 2, ch. 2/b, § 91 (1901) [ed. Elisabeth Förster-Nietzsche] [tr. Kaufmann/Hollingdale (1967)]
    (Source)

(Source (German)). Alternate translations:

Perhaps I know best why man is the only animal that laughs : he alone suffers so excruciatingly that he was compelled to invent laughter.
[tr. Ludovici (1910)]

Perhaps I know best why it is man alone who laughs; he alone suffers so deeply that he had to invent laughter.
[Common, e.g.]

Perhaps I know best why man alone laughs: only he suffers so profoundly that he was bound to invent laughter.
[tr. Hill/Scarpitti (2017)]