Abandoned by his father and brother when he was extremely young, Willy is left materially and emotionally ungrounded. However much he fears abandonment himself, he made his son Biff feel emotionally abandoned when Biff discovered Willy's secret affair. Willy's powerful fear of abandonment drives him to form unrealistic expectations for and obsess over his sons. When Biff found out about Willy's affair, he did in fact abandon his father and pretty much disappeared for many years. Willy permanently abandons his son and family at the end of Death of a Salesman by committing suicide. Ironically, this final decision on Willy's part was a final attempt to connect and give something to his son.
Contrary to Willy assumptions, Biff does not emotionally abandon his father even after he discovers the affair.