If you are lonely when you’re alone, you are in bad company.

[Si vous êtes seul quand vous êtes seul, vous êtes en mauvaise compagnie.]

Jean-Paul Sartre (1905-1980) French philosopher and writer

(Attributed)

Widely attributed, in both languages, to Satre (especially by self-help books), but I am unable to find any actual citation of its source. Given that Sartre wrote a lot about loneliness, it is possibly a paraphrase of of something else he said.