As per Enotes policy, I edited your original question to reflect only one inquiry.
The relationship between Happy and Willy Loman is an interesting topic to analyze. Happy (Harold) Loman is the youngest of Willy's two boys. Far from obtaining the usual concessions that younger children get due to their age, Happy has been relegated to a basic role as his big brother Biff's shadow. Biff, who is his own father Willy's pride and joy, is nevertheless very different from Willy; the only reason why Willy and Biff bond during Biff's high school years is because Biff also served as a sort of puppet that served all of his father's whims.
Surprisingly, however, it is Happy who resembles Willy the most and, ironically (though perhaps not entirely coincidentally), he is the most ignored of the two brothers regardless of his mild successes.
It is clear that Happy has remained stuck in the place in time during his childhood where he would try to emulate his brother's desire to do as Willy wishes. Contrary to Biff, however, Happy still tries very hard years later to still please Willy by creating a facade of success, and by constantly pointing out how he has a girlfriend, or that he will get married, or even that he has lost weight. Always ignored, Happy is never held accountable, nor is he held responsible for nearly anything he says or does. For this reason, continues his life in his "happy-go-lucky" ways.
Since Happy was raised without clear and solid goals he is, as he himself points out himself, "blown full of hot air". He realizes that he has no idea why he does anything. He lacks any capacity for in-depth analysis, goes with the flow with just about anything, dates women in whom he is no longer interested and, basically, lives his life to briefly satisfy his ego.
There are a few redeeming qualities in Happy. He is self-sufficient as he does have a job and an apartment. He seems caring enough for his family but not so much that he would feel responsible to help them out financially. His main issue remains the same: he cannot anchor himself in reality. As a result, he continues to dream about following Willy's own empty dream of business success, and continues to create stories about himself, and Biff. This is what makes him so similar to Willy: they are quite happy to live a fantasy. Therefore, that is mainly the reason for his lack of success: a dysfunctional upbringing combined with an inherited and well-learned tendency to create reality rather than learning from it.