How you express yourself shows the kind of person you are - rude or considerate, selfish or generous. The really polite person is tuned in to other people's feelings and can put themselves in another person's place. They can understand how it would feel to be new to the neighborhood or job, or what it's like to be the shortest person in the class or the shyest person at a party. They react with understanding and with the kind of manners that matter because they come from the heart.
Treating others the way you would like to be treated is the easiest rule to follow and encourages others to treat you in kind. Even the most polite people in the world can occasionally say the wrong thing or make mistakes, but being kind, considerate, and generous on a daily basis shows true manners. We can't police ourselves all the time, but before you say or do something, ask yourself how you would feel if others said or did that to you. The more you practice this rule, the more natural and easier it becomes.
Good manners show the best you have to offer and encourage others to be their best. Practicing these manners on a daily basis makes for a more pleasant life.