“I can understand companionship. I can understand bought sex in the afternoon, but I cannot understand the love affair.”

Yo-Yo Boing! (Spanglish novel, 1998)
Variant: We have a similar problem. I can understand English, but I cannot understand Americans.

Act 1
The Devil and the Good Lord (1951)

Canzone 105, st. 2
Il Canzoniere (c. 1351–1353), To Laura in Life

Although this quote has been attributed to Wittgenstein in Wittgenstein's Mistress by David Markson, there is no verifiable source from Wittgenstein that it can be traced back to.
Disputed

"The Case for the Ephemeral"
All Things Considered (1908)
Context: I cannot understand the people who take literature seriously; but I can love them, and I do. Out of my love I warn them to keep clear of this book. It is a collection of crude and shapeless papers upon current or rather flying subjects; and they must be published pretty much as they stand. They were written, as a rule, at the last moment; they were handed in the moment before it was too late, and I do not think that our commonwealth would have been shaken to its foundations if they had been handed in the moment after. They must go out now, with all their imperfections on their head, or rather on mine; for their vices are too vital to be improved with a blue pencil, or with anything I can think of, except dynamite.
Their chief vice is that so many of them are very serious; because I had no time to make them flippant. It is so easy to be solemn; it is so hard to be frivolous.

Source: The new world of Brunello Cucinelli https://www.morningfuture.com/en/article/2018/05/02/beauty-hope-brunello-cucinelli-work-future-digital/276/ Morning Future, 2 May 2018

Source: The Metamorphosis (1915)