Human beings reveal their character most clearly by what they find ridiculous.

[Durch nichts bezeichnen die Menschen mehr ihren Charakter als durch das, was sie lächerlich finden.]

Johann Wolfgang von Goethe (1749-1832) German poet, statesman, scientist

Elective Affinities [Die Wahlverwandtschaften], Part 2, ch. 4, “From Ottilie’s Journal [Aus Ottiliens Tagebuche]” (1809) [tr. Hollingdale (1971)]
    (Source)

(Source (German)). Alternate translation:

There is nothing in which people more betray their character than in what they find to laugh at.
[Niles ed. (1872)]

Men show their characters in nothing more clearly than in what they think laughable.
[Source]

For an opposite perspective, see Lichtenberg.