After the first test, Joseph has already learned a lot about his brothers.
He knows that they’re genuinely remorseful about selling him to Midianite traders, which resulted in spending the majority of his life in Egypt. He knows that Reuben had tried (in vain) to stop them from harming Joseph. He also knows that they are willing to risk their life to save the life of Simeon, who Joseph imprisoned to force them to bring Benjamin.
Now, only one question remains: Would they abandon their little brother again?
The Bible goes through great pains to demonstrate the closeness of Joseph and Benjamin. You could explain that way by saying that he was the only full brother of Joseph (since both were sons of Rachel). I think that undersells the point a tad, though.
Like Joseph, Benjamin was one of the youngest (to be more specific, Benjamin was the youngest, whereas Joseph was the second youngest). Given his age, it’s possible that Joseph simply felt protective of Benjamin and wanted to ensure he was taken care of.
But when you turn the clock back a bit to Genesis 30, you remember that there was a lot of tension between Rachel and Leah as a result of their childbearing. Both were barren (for a time) and forced to watch as their maids were given to their own husband as wives, who, in turn, produced children.
It’s entirely possible that this type of rivalry extended even unto their children. The brothers sold Joseph into slavery because they hated his dreams, but also because he was their father’s favorite (Genesis 37:4). He was his father’s favorite because he was the oldest child of Jacob’s favorite wife, Rachel.
With Joseph out of the way, who now has the bullseye on their back? The only other son of Jacob and Rachel, which is Benjamin.
Regardless of the why, it’s shown throughout Genesis 43-44 that Joseph wants to honor Benjamin. He wants to see if that honor will result in jealousy from his brothers, and like it did with Joseph, his exclusion from the family.
That’s the basis of Joseph’s second test for his brothers. By exalting him in Genesis 43 with a double portion of food and then framing him for royal theft in Genesis 44, he produces a scenario that will test the brothers’ love for Benjamin.
