As Joseph tested his brothers, he showed them the philosophy by which he operated,

Then Joseph said to them the third day, “Do this and live, for I fear God…” (Genesis 42.18).

Joseph was not being arrogant, nor was he self-righteous, but he showed that he believed there is a God in heaven, and that he knew he would answer to that God. Therefore, Joseph wanted his brothers to know the basis for what he was doing and why he was testing them.