Why does Figaro become jealous of Susanna in Act IV?

In Act IV of Mozart's opera The Marriage of Figaro, Figaro becomes jealous because he believes Susanna is unfaithful to him after learning of a secret tryst.1

The misunderstanding begins when Barbarina, a young girl in the castle, reveals that she lost a pin given to her by Count Almaviva to deliver to Susanna as a token of their planned meeting. Upon hearing this, Figaro concludes that Susanna is meeting the Count behind his back. His suspicion is further fueled when he observes the Count flirting with a woman he believes to be Susanna in the garden, unaware that the woman is actually the Countess in disguise.1

Throughout the act, Figaro remains in hiding, observing these interactions and lamenting the perceived betrayal, while Susanna—who is aware of his presence—deliberately plays along with the ruse to teach him a lesson for his jealousy.1