When it comes downt to it, there are two ways to get people to do things: by word or by deed. Or, to put it another way, I can ask you to do something or I can make you do something. This lesson is particularly a good one to learn as a child (I have no particular age in mind here, but the sooner the better, for both parent and child). The first time this truth arose in our home was when one of my sons had been ignoring the verbal pleas of my other son to "please stop" whatever it was that he was doing. After an exasperating few minutes, the offended son approached me to settle the dispute. When I confronted the offending son, our conversation quickly turned to what this kind of foolish negligence on his part will lead. Today, his brother; tomorrow, his friend; the next day, someone not so friendly who won't ask you twice. And there's the rub.
To be sure, this is only one side of the equation: other people ought neither to ask nor compell us to do things that are either wrong or against our conscience. However, that's for another post (well, series of posts really). Scripture says, "As far as it depend on you, be at peace with all men." Part of what this means is, "Don't ignore people when they speak to you," particularly if it's neither illegal nor immoral. Of course, it goes without saying (or does it?) that we are always free to turn down a request, to acquiesce, to decline, etc. But the point is, "Don't ignore people."
Now, merely acknowledging people doesn't guarantee peace; the other person might not like the way you choose to acknowledge them. But what will ensure hostility is prolonged negligence: nothing infuriates people more than being ignored when it's clear to everyone, especially them, that they are being ignored.
And this brings us to my point to my son. My efforts to train him to acknowledge his brother at the age of 8 (whether 'yea' or 'nay') is to prevent him from being beaten to a pulp someday by someone he's chosen to ignore, whether by fear, indifference, malice, or ignorance. This is not necessarily the same as cowering through life in fear of offending people. It is, on the other hand, at the heart of giving respect to other human beings made in the image of God. Whether we use speech and language rightly (as in, "Please pass the salt") or wrongly (as in, "Give me the salt, you blockhead!"), we are speaking; we are exercising part of our divine image-bearing essence. To communicate is to be like God. Whether we do it rightly or not is a function of our soul's relation to God: if we are born again, our speech may be holy speech; if we are not, it is fallen speech. Either way, though, we are imaging forth what God does: communicate himself to others.
In the end, I want my son to be trained in the habits of respect and dignity: answer people when they speak to you. Decline their offers or requests if you like and if it's right, but do not ignore them.