Willy's life took its most obvious turn when he was caught cheating on his wife with another woman by his son Biff. It destroyed their father-son relationship, which was going pretty well up to that point. Biff loved his mother very much, respected her greatly, and could not fathom why his father would betray her in such a way. Biff was heartbroken, and wept when he discovered his father's treacherous actions. It came at an inopportune time for Willy too, because he'd rested many of his hopes and dreams on his son. Biff was headed to the University of Virginia before he'd flunked math and gone to talk to his father about it in Boston. He'd gone to discuss summer school and having Mr. Loman talk to his math teacher when he discovered another woman in his father's bathroom.
Although most of the book seems to revolve around Willy, and he appears to be the main character of the play, the heart of the play, its main issues, what can be called the "meat" or substance of the play, is rather centered on his son, Biff. Willy remains the same throughout the play, what is called a static character, because he undergoes little inner change, while Biff is the dynamic character, undergoing important inner change throughout the course of the play. Biff discovers who he is. He is NOT all that his father built him up to be, a successful business man who is well-liked by everyone and thought highly of. He is a man of the outdoors, a man not meant for the office, a simple man. (Granted, he is also a thief, but that is an unfortunate side quality, and there is definitely some complicated reasoning behind that.) This discovery about himself leads him to be completely honest with his father, and Willy does not know how to react. He breaks down and wants to run from Biff's words, but Biff doesn't let him. Disappointed that his son will never become what he wanted him to, Willy Loman is distraught. He does not know where to turn. The climax of the play occurs during the big argument between Willy and Biff, in which Linda and Happy are around to witness. Here, Biff lays down the rubber tube from the gas pipe on the table, determined to address every issue between them, including addressing the fact that Biff had been under arrest for a couple months out West. He no longer wants to fight with his father and thinks that laying everything out on the table will help solve their previously unspoken issues, yet Willy is in denial. He does not want to listen and tries to escape. Biff eventually breaks down completely, in tears, and Willy finally comes to realize that his son does indeed love him. After this rather climatic event, everything appears to relax and run smooth again.
Biff's self-realization creates a fair amount of drama in the play, one, because he came to this realization in the middle of running away from the office of the man he'd just stolen from and two, because telling his father caused Willy to inform the boys that he was fired. It came as a shock to Biff and Happy, who know they now have to step up and help out their family more with household income. It further contains elements of drama in that it goes against everything Biff was brought up believing, all the brain washing Willy put Biff through, all the hopes and dreams laid on him even as a young child and throughout his teens. He'd always felt guilty for not going into business like his father wanted him to, but it appears that when he came to his self-realization, Biff was able to shrug the weight of this guilt off and accept himself for who he knows himself to be.