Jack is not as handsome or as clever as his older brothers, but he is kind. The title of this traditional third-son-makes-good as almost a misnomer; although Jack is reasonably brave, it is his kindness that characterizes him and allows him to succeed on his rather accidental quest.
After his older brothers fail to return from seeking their fortunes, Jack's unloving mother sends him to find them. As is often the case in stories of this sort, Jack performs acts of generosity or kindness towards various inhabitants of the natural and supernatural worlds which will pay off during later challenges. Jack is almost ego-less. He seems to think nothing of his own good deeds, and even when he hears about a princess available for marriage to whomever can pass three tests, he does not consider winning her but only deduces that this is where his brothers have gone. After he meets the princess, he is motivated to accept the impossible tasks not by love but out of pity for her imprisonment.
At this point the initiative in the story actually shifts to the princess, whose knowledge and initiative help Jack to succeed and guide their escape from the evil magician. She also takes the romantic lead in their relationship. Although Jack is the hero of the tale, the princess supplies talents and personality traits which complement his -- a rather wonderful depiction of marriage and governance.