Good manners are not the most common quality, yet have always been stressed as important. I would like to explore a far more valuable reason to have impeccable manners other than just to look good to others.
Good manners are a show of respect to other people, as does following a code of ethics and moral conduct. http://ethicalism.org is a good guideline. They are all absolutely identical, but not quite the same.
To have respect requires being constantly very present and attentive to how your every word and actions will affect other people and your environment, in other words, you have to be awake.
Humans live in a conscious sleep. Can you recall what you saw on your drive to work today? You made it without an accident, yet you probably cannot remember much if anything of what you saw. That is just an example, but you can take any section of any day and when you try to recall it, you will realize you slept right through it. Your body functions automatically while your mind is asleep.
In order to develop yourself spiritually, or even just as a better human being, you need to be awake, at least more awake, or you could say; be present, more than you currently are.
Forcing yourself to practice good manners at all times is a way to wake up. The more present you are, the more you will see things that you currently miss while in your state of conscious of sleep.
You will see how you affect other people which will prevent insulting or upsetting people, it will help smooth over problems as you will see them just as they start, and you will be more present to see opportunities that are now as if they did not come to you, while you complain you never get a break.