If we only wanted to be happy, it would be easy; but we want to be happier than other people, which is almost always difficult, since we think them happier than they are.

[Si on ne vouloit qu’être heureux, cela seroit bientôt fait; mais on veut être plus heureux que les autres; et cela est presque toujours difficile, parce que nous croyons les autres plus heureux qu’ils ne sont.]

Charles-Lewis de Secondat, Baron de Montesquieu (1689-1755) French political philosopher

Pensées Diverses [Assorted Thoughts], # 978 / 1003 (1720-1755)
    (Source)

Also found as "Si on ne vouloit ’être qu'heureux ...."

This is sometimes mistakenly sourced to his Persian Letters [Lettres Persanes] (1721).

(Source (French)). Other translations:

If one only wished to be happy, this could be easily accomplished; but we wish to be happier than other people, and this is always difficult, for we believe others to be happier than they are.
[E.g. (1891)]

If we only wanted to be happy, that would soon be done. But we want to be happier than others, and this is almost always difficult because we believe others to be happier than they are.
[tr. Clark (2012)]