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.