They had been classmates, roommates, in the State University, but always he thought of Paul Riesling, with his dark slimness, his precisely parted hair, his nose-glasses, his hesitant speech, his moodiness, his love of music, as a younger brother. (4.3.6)
Babbitt thinks of Paul Riesling as a younger brother whom he needs to protect and look out for. The problem is that Babbitt doesn't realize how much he relies on Paul to keep his own life together. The moment Paul goes to jail and disappears from his life, Babbitt falls to pieces because he no longer has anyone else to look out for. He needs to face himself, and he doesn't really like what he sees.