I hasten to laugh at everything, lest I should have to weep at everything.

[Je me presse de rire de tout, de peur d’être obligé d’en pleurer.]

Pierre Beaumarchais (1732-1799) French playwright, polymath [Pierre-Augustin Caron de Beaumarchais]

The Barber of Seville [Le Barbier de Séville], Act 1, sc. 2 [Figaro] (1773) [tr. 1896]
    (Source)

(Source (French)). Alternate translations:

I make haste to laugh at everything for fear of being obliged to weep.
[Motto for the London Figaro (1871)]

I am eager to laugh at all for fear of being obliged to weep.
[Source (1887)]

I hasten to laugh at everything for fear that otherwise I might be forced to weep over it.
[tr. Taylor (1922)]

I force myself to laugh at everything for fear of being forced to weep at it.
[tr. Bermel (1960)]

I forced myself to laugh at everything for fear of having to weep.
[tr. Wood (1964)]

I always hasten to laugh at everything for fear that I may be obliged to weep.
[tr. Luciani (1964)]]

I make a point of laughing at everything, for fear of having to cry.
[tr. Anderson (1993)]

I make a point of laughing at life, because otherwise I'm afraid it would make me weep.
[tr. Coward (2003)]

I quickly laugh at everything, for fear of having to cry.
[Bartlett's]

And endless other variations ("I force myself to laugh at everything, for fear of having to cry") in one-off passages.

Sometimes given, in French, as "Je me hâte de me moquer de tout, de peur d'être obligé d'en pleurer."

Compare to Byron (1820).