When the boys were younger Willy placed many of his dreams into his son Biff. He looked at Biff's athletic abilities and probably lived his dreams through Biff's accomplishments. He was on the road often as a salesman but went his sporting events when he could.
Willy's relationship with Happy was very different. Happy was often overlooked. He loved his father and would do anything to seek out his approval and attention such as bringing it to his attention that he had lost some weight. He continues to seek his father's approval even as an adult.
Willy's relationship with Biff is crushed once Biff catches his father in the hotel room with a female that is not his mother. He sees that Willy has given her the stockings intended for his mother. This sets Biff back emotionally and places a wedge between his relationship with his father.
When they were younger, Willy had many dreams instilled in these boys. He believed that Biff would be the star player and while he dotes on Biff, Happy is overlooked at most times. Even as an adult we see Happy trying to get his mother and father's attention and to be noticed. Even at the end of the play Happy wants to carry out Willy's dream. Biff however gets shattered when he sees his father with "The Woman" and loses respect for his father then and there. This places a rift between them and hinders the entire relationship of the family.