Simba initially fights with Nala, but they soon realize they are friends. She tells him of Scar's reign, which begins Simba's quest to gain his father's kingdom back from Scar.
William Wallace in Braveheart only begins to fight after his wife, Murron (on the right), is killed by the local British commander. Her death haunts him throughout the movie in addition to sparking the beginning of his quest for freedom.
King Arthur began his quest by drawing the sword from the stone (not Excalibur). Not traumatic of course, but surprising because Arthur was a lowly squire at the time.
Luke abruptly began his quest to fight the Empire when Stormtroopers murder his aunt and uncle. Luke left the farm and never returned, going on to become a Jedi master.