Jacob had always shown favouritism. First, it was Joseph: Jacob doted on him, made him a special coat (Gen. 37:3), and generally made it clear that he loved Joseph above all his other sons. However, even after he believes Joseph has been killed, he doesn't learn his lesson, but instead begins to dote on his youngest son (and Joseph's only full brother) Benjamin. No matter how much his other sons try to convince him that only by bringing Benjamin to Egypt will they have their names cleared, Jacob refuses. Even when Reuben pledges the lives of his own two sons against Benjamin's safety (Gen. 42:37), Jacob refuses. Benjamin is staying at home, no matter what happens to his other sons.
It almost makes you wish there were self-help parenting books back then.