What turned Happy into the kind of man that he is in Death of a Salesman?

Happy behaves like a typical middle child, although he is
the youngest and the "baby" of the house. What this means is that he is consistently
causing trouble to call attention to himself, or simply seeks the attention through
different means. The reason why Happy turns out the way he does has everything to do
with two main things: First, the fact that he grew up under the shadow of Biff-the
family's favorite and, second, that he was enabled to behave whichever way he wanted
because he was often left to his own devices. Idle hands are the devil's companion and
Happy was idle most of his youth. Idle, and ignored in favor of
Biff.

We see in the play that Happy always tries his best
to stand out, but is not lucky at it. He consistently calls out his deeds to his
parents. He would tell them in one occasion that he has lost weight and then later on he
would say that he is getting married.

We know that the
typical Loman fashion is to embellish the truth, or deny it, and then come up with a new
one. Therefore, Happy is well-versed in the art of coming up with stories and lies.
Since he is so good at creating them, he uses them to pick up
women.

This is indicative of another tendency in among the
Loman males: Their treatment of women. They tend to want to control situations by
blowing their own egos out of proportion. Happy's behavior with women is no different
than Willy taking a mistress when Biff and Happy were in high school. Womanizing seems
to be second nature to the Lomans.

So, in summary Happy
turned out the way he was allowed to turn out due to a parenting style that did not set
in limitations,and because of the natural tendency of the Lomans to want to bully,
control, and create fantasy lives.