Because it reflects our personality and most of the people judge us by our manners and how we take it in ourselves.This is the main aim of Education.
Hope it helps and pls mark as the best answer. :-)
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Good manners are important because it leads u to a prosperous life.
Those with bad manners are not praised. Only those who lived with good manners are praised. The best example for this, is Dr. Abdul kalam. He is such an inspiration for the youths, and justified that, good manners are important to live even after u die, with ur thoughts.
Therefore, I say that, good manners are important to lead a happy life.
I had given the answer for why, and justified "how", by giving the example of Dr.Abdul kalam. 
Hope this will help u.