A donkey once went out on hire, and upon seeing onions, it became sweet.

This couplet humorously suggests a surprising transformation. It describes a stubborn donkey, often associated with a difficult nature, being hired out. But then, upon seeing onions, something unexpected happens: it 'becomes sweet.' This isn't literal sweetness; rather, it implies that the donkey, known for its strong will, surprisingly mellowed down, became agreeable, or changed its usual behavior when confronted with something as pungent and common as onions. It's a playful way to talk about how circumstances, even unusual ones, can lead to an unexpected change in someone's disposition or stubbornness.