Why are aphorisms cynical more often than books are for instance?

A good single sentence saying can’t require background evidencing or further explanation. It must be instantly recognizable as true. It also needs to be news to the listener. Most single sentences that people can immediately verify as true they already believe. What’s left? One big answer is things that people don’t believe or think about much for lack of wanting to, despite evidence. Drawing attention to these is called cynicism.

HT to Robin Hanson for the question and to Francois de La Rochefoucauld for some examples:

We often forgive those who bore us, but we cannot forgive those whom we bore.

We promise according to our hopes; we fulfill according to our fears.

What often prevents us from abandoning ourselves to one vice is that we have several.

We confess to little faults only to persuade ourselves we have no great ones.

There are few people who are more often wrong than those who cannot suffer being wrong.

Nothing prevents us being natural so much as the desire to appear so.