The straw that broke the donkey's back

Once upon a time, there were two merchants who had a donkey. They needed to transport some goods to a nearby town. The first thing they loaded onto the donkey was a bale of straw.

"Ah, my friend," said one merchant to the other, "we have all these things to sell, and it is so far away, and look at this donkey, it is strong, it can certainly carry more."

The other merchant, who never spoke, nodded, and proceeded to load a collection of vases and bowls onto the donkey's back.

"My friend," said the first merchant, "I still think the donkey can surely hold more than this!"

The other merchant nodded, and loaded the vases and bowls with some of the grain they were going to sell as well.

For an hour, this continued; the first merchant would notice that the donkey was strong, and could surely hold something more, and the second one would burden the beast with yet another item. Eventually, the donkey was shaking under the load, but all of their goods were loaded on top.

The first merchant then noticed that when they had first loaded the bale of straw, however, one single piece had fallen out. "No sense in wasting even a single piece of straw. Friend, pick that up."

For the first time in ages, the second merchant spoke. "But the donkey - it is quaking under the load. It will surely collapse if we load it up with anything more!"

Bewildered more by his associator's rebellion than by the fact he had spoken up, the first merchant frowned, and picked up the piece of straw on his own, and placed it on top of the load.

"And it was the last straw which broke the donkey's back"

For a few years ago, I read this fable and I don't understand the felling of the donkey. I ever tries to find the answer about this question that why the last straw can broke the donkey's back? but I never found it. Until now, I get all of the answers of that question and I am so happy when the donkey broke the back because from now the donkey is no longer to carry anything. Thank you to someone who put the last straw on the back....