The Heart Is a Lonely Hunter is hardly full of happy characters. No, the people in this Georgia town have some serious woes, and many of those woes stem from their general dissatisfaction with their lives. Isolated, frustrated, and discombobulated, these characters struggle to come to terms with the things that keep them down, like poverty, gender, race, and class. Many of these characters feel like outsiders, which means they will never feel the satisfaction of belonging, so they turn to isolation, strangely, as a means to cope.
Biff isn't dissatisfied at all. While he's sad and lonely, he is mostly content with his choices.
Singer sets himself up for failure. He never allows himself to be satisfied without Antonapoulos, but he wasn't content with Antonapoulos either. Talk about a catch-22.