Symbolism, Imagery, Allegory
Despite all of his flaws, Rousseau generally thinks of himself as a good guy. Sure, he has a weakness for the women, but he doesn't often act maliciously towards others. Just don't remind him of the Ribbon Incident of 1728.
Who, Me?
When Rousseau's guardians lose a little pink ribbon, the last person they think to blame is Rousseau. Rousseau doesn't even provide any motivation for stealing the ribbon. He simply admits that it was "quite old" (2.1.89). But when the ribbon turns up in Rousseau's possession, he has a bit of backtracking to do.
Rousseau points the finger at Marion, an innocent servant who's always been kind to him. Everyone thinks of her as a "good girl, sensible and absolutely trustworthy" (2.1.89). Rousseau admits to himself (and all of his readers) that if he were brave enough, he'd totally confess and save Marion the trouble.
A New Rousseau
Why does Rousseau still remember this incident, all these years later? He's turning over a new leaf, so to speak. Old Rousseau would lie through his teeth to protect himself from punishment, even at the expense of an innocent servant. New Rousseau goes out of his way to confess his crime, even though it likely has no significance to his readers. No stone will be left unturned in Rousseau's confessions—not even a paltry little ribbon escapes notice.