If you are courteous, your good manners show friendliness and concern for others, like your courteous habit of holding the door for people entering a building with you.
The adjective courteous comes from the Old French word curteis, which means “having courtly bearing or manners.” Courtly described the court — nobles who hung around the castle, the entourage of kings and queens. Their refined manners, not to mention their wealth and power, set them apart from the masses. So courteous behavior is a reminder of the value of good manners.