Laudator Temporis Acti

St. Augustine, Confessions 2.9.17 (tr. William Watts, rev. W.H.D. Rouse):

Is it for that no man doth so readily laugh alone? Ordinarily indeed nobody does; but yet a fit of laughter sometimes comes upon men by themselves and singly, when nobody else is with them, if anything worthy to be laughed at comes either in their eye or fancy.

an quia etiam nemo facile solus ridet? nemo quidem facile, sed tamen etiam solos et singulos homines, cum alius nemo praesens est, vincit risus aliquando, si aliquid nimie ridiculum vel sensibus occurrit vel animo.

James O'Donnell in his commentary on St. Augustine's Confessions (at least in the web version) has no note on solitary laughter.

Cf. Diogenes Laertius, Lives of the Philosophers 1.9.107-108 (on Myson, tr. R.D. Hicks):

Aristoxenus in his Historical Gleanings says he was not unlike Timon and Apemantus, for he was a misanthrope. At any rate he was seen in Lacedaemon laughing to himself in a lonely spot; and when some one suddenly appeared and asked him why he laughed when no one was near, he replied, "That is just the reason."

Ἀριστόξενος δέ φησιν ἐν τοῖς σποράδην (Wehrli ii, frag. 130) οὐ πόρρω Τίμωνος αὐτὸν καὶ Ἀπημάντου γεγονέναι· μισανθρωπεῖν γάρ. ὀφθῆναι γοῦν ἐν Λακεδαίμονι μόνον ἐπ' ἐρημίας γελῶντα· ἄφνω δέ τινος ἐπιστάντος καὶ πυθομένου διὰ τί μηδενὸς παρόντος γελᾷ, φάναι, "δι' αὐτὸ τοῦτο."

Arthur Schopenhauer, Parerga und Paralipomena, ch. XXVI (Psychologische Bemerkungen, tr. T. Bailey Saunders), refers to this anecdote about Myson:

I am not surprised that some people are bored when they find themselves alone; for they cannot laugh if they are quite by themselves. The very idea of it seems folly to them.

Are we, then, to look upon laughter as merely a signal for others—a mere sign, like a word? What makes it impossible for people to laugh when they are alone is nothing but want of imagination, dullness of mind generally—ἀναισθησία καὶ βραδυτὴς ψυχῆς, as Theophrastus has it. The lower animals never laugh, either alone or in company. Myson, the misanthropist, was once surprised by one of these people as he was laughing to himself. Why do you laugh? he asked; there is no one with you. That is just why I am laughing, said Myson.