What does Arthur's behavior immediately after pulling the sword free tell us about him?

Arthur has no idea he is pulling the sword from the stone that will make him king of England. He is with his family at a Christmas joust, and his brother Kay has lost his sword. Kay sends Arthur to get another sword, but when he gets home, there is nobody around to help him because they are all at the joust. Annoyed, Arthur sees the sword in the stone and easily pulls it out, thinking it will do for Kay. He has no idea that nobody else can do this.
When Kay gets the sword, he immediately recognizes it as the famed sword in the stone. He runs to tell his father. When his father questions Arthur, Arthur explains how he casually pulled the sword from the stone. To him, it is no big deal. He doesn't understand what all the fuss is about. When his father tells him he will be king, Arthur asks why (wherefore means why):

Now, said Sir Ector to Arthur, I understand ye must be king of this land. Wherefore I, said Arthur, and for what cause? Sir, said Ector, for God will have it so; for there should never man have drawn out this sword, but he that shall be rightwise king of this land.

Arthur takes this all in stride. He has no ambition to be king, unlike others who have tried in vain to pull out the sword. This shows that he is a modest and humble person who doesn't want power and who has no desire to lord it over other people. This suggests he will be a good and wise king.