Thought
We are not told exactly why Joseph tested his brothers by hiding the cup in the sack that belonged to Benjamin, but we can make a good guess based on what we know of his story. Joseph was a good man and had proved himself to be a man of honor, despite what his family did to him. Joseph was also a powerful man at the time his brothers came to him in Egypt, and he certainly could have forced his brothers to say or do whatever he wanted. And we also know that Joseph was a man who had suffered much at the hands of his brothers. Perhaps through this test that he designed, he wanted to see how his brothers behaved—would they turn on one another, as they had turned on him so long ago, and give up the youngest of their family? Or would they behave with honor and loyalty?