He is labeled mad for acting against convention, but Diogenes points out that it is the conventions which lack reason.
Diogenes is a harsh critic of Plato, regularly disparaging Plato’s metaphysical pursuits and thereby signaling a clear break from primarily theoretical ethics.
One guiding principle is that if an act is not shameful in private, that same act is not made shameful by being performed in public. For example, it was contrary to Athenian convention to eat in the marketplace, and yet there he would eat for, as he explained when reproached, it was in the marketplace that he felt hungry. The most scandalous of these sorts of activities involves his indecent behavior in the marketplace (masturbation), to which he responded “he wished it were as easy to relieve hunger by rubbing an empty stomach” (Diogenes Laertius, Lives of Eminent Philosophers, Book 6, Chapter 46).
As a homeless and penniless exile, Diogenes experienced the greatest misfortunes of which the tragedians write, and yet he insisted that he lived the good life: “He claimed that to fortune he could oppose courage, to convention nature, to passion reason”
"Man is the most intelligent of animals -- and the most silly."
“When I look upon seamen, men of science and philosophers, man is the wisest of all beings; when I look upon priests and prophets nothing is as contemptible as man”
“Diogenes, when asked from what country he came, replied, "I am a citizen of the world”
"Of what use is a philosopher who doesn't hurt anybody's feelings?"
Internet Encyclopedia of Philosophy