Why is it said that you should never test human nature?

Is it really true that all human nature cannot withstand testing?

There was an older brother, naturally hospitable, with many friends, and a great cook.

One day, on a whim, he decided to cook himself and invited friends over for a meal, boldly saying, "Whatever you want to eat, just order it. There's nothing I can't make, as long as it's not something you're afraid to eat."

Upon hearing this, a friend casually said, "We want to eat swan meat."

He thought it was just like a toad wanting to eat swan meat—a joke. But on the day of the meal, there was indeed a hard dish on the table that matched that description.

After three rounds of drinks, the police came knocking, saying they had received a tip that the older brother was suspected of killing a wild goose protected at the second level by the state.

And the tipster was none other than the older brother's girlfriend.
That swan was a wild goose, also called a gray swan, or a wild goose.

The police investigation found that the wild goose was farmed legally, with complete documentation from the seller, making it a lawful business. It was legal to serve it on the dining table.

This meal made the older brother feel incredibly disheartened. "I paid for everything myself, spent hours busy preparing and cooking it, kindly invited you all to eat, hoping you’d enjoy and be happy. But just after we finished eating, I was reported. How can I have friends like this?"

Angrily, the older brother decided to reveal the whole story.