"Judah is a lion's cub, from the prey, my son, you have gone up."  Gen 49:9

A righteous person is one who rules over himself and all his impulses.  Such a person is worthy of being a ruler over others, because he will rule over them with the same righteousness as he rules over himself. 

The Rabbis comment that Judah elevated himself by two actions.  He stopped his brothers from killing Joseph and he publicly embarrassed himself to save Tamar. This was the reason Judah was chosen to be the tribe of the future kings of Israel.  In saving Joseph he demonstrated that he could rule over himself and not be influenced by the behavior of others. He was so self-confident that his brothers listened to him.  Similarly, when he willingly caused himself public embarrassment it took much self-discipline.

This is the power of ruling over oneself.  Such a person can be a ruler because of his own inner strength and not merely because other people happened to choose him. This ability to rule himself made Judah a true king.