Adler had a handful of theories about why we are the way we are. Major ones are inferiority, parenting styles, and birth order.
Adler believed that we all begin with feelings of inferiority as children, and strive to overcomes these feelings our entire lives. Some people are particularly driven to overcome these feelings and strive to be very successful and powerful. This theory would argue that our most powerful leaders (presidents, religious leaders) are driven by a need to be superior in order to negate their inferior feelings. If you think about it, one could analyze W. Bush by saying that he was driven by a need to become president to shake off the feeling that he was the least successful and dumbest of the Bush dynasty. And now he's the least popular president in the history of popularity polls. That can't be good for the inferiority complex.
Parenting styles refer to how parents protect their children from the world. If a parent shelters their child from even the knowledge of bad or dangerous things in the world, when the child grows up and discovers them, the child will regress and seek the same shelter they had with their parents. This could manifest in living with their parents and refusing to 'grow up' or finding a significant other to act as a surrogate parent. When parents do not shelter their children from any of the bad or dangerous things in the world the children grow up to distrust everyone and have difficulty forming healthy relationships. Adler recommends parents do not shelter their children from the bad in the world, but do protect them from danger.
Lastly, Adler has a lot to say about birth order. He believes that the oldest child originally believes they are the center of their parents' world, and when the second child is born, this theory is shattered. The oldest child resents the younger child, feels inferior, and competes for love and attention. The older child is also under pressure from the parents to be responsible for the younger child. This theory states that the oldest child will be the one with the most emotional problems. The youngest child is the child that is most doted on by the parents', so that child is most likely to grow up feeling inferior to others, believing they are not capable of taking care of themselves. According to Adler, the middle child is the one who will be the most emotionally stable. Coincidentally, Adler was a middle child.
Robbins, S.P., Chatterjee, P., & Canda, E.R. (2006). Contemporary human behavior theory: A critical perspective for social work (2nd ed.). Boston: Pearson.