Theseus is one of the great heroes of Greek mythology. His exploits demonstrate his intelligence, sense of justice, and great physical skill. Theseus is thought to be the one who united Attica (region of Greece where Athens is), showing him to be a great king and politician. Civilization conquering barbarism and unnaturalness was an important theme for the city of Athens, so of course her greatest hero would bring justice, and order to the world. Theseus is truly the perfect hero for Athens, the center of ancient Greek culture and academics.
The bull shows up repeatedly in the story of Minos and Theseus. Europa is abducted by Zeus in the form of a bull. Pasiphae is forced to lust after a bull. The Marathon bull kills Minos' son, and the Minotaur in the labyrinth is part bull.
Theseus metes out justice to each of the six foes he faces on his way to Athens. Theseus makes the punishment fit the crime. Literally.
Theseus triumphs over unjust and savage beings throughout his story. He defeats the bad guys at every turn, from his "Six Labors" on the road to Athens, to the wild Marathon Bull, the unnatural Minotaur, and beyond.