Willy's high expectations for his sons interferes with their abilities to form lasting relationships. Biff sees the reality of Willy's life, and wants to distance himself. Happy is content to live in Willy's delusions. Neither situation is conducive to dating and marriage.
Additionally, neither Biff nor Happy hold down satisfactorily paying jobs. Willy has set them up for failure in business and in love. By living in a fantasy world and pulling his sons into it, only Linda really lives with the daily rational truth.
Both Happy and Biff are lonely and bored, but neither has the capacity to declare independence and follow his own path. It's part of the tragedy of Willy Loman.