One of the reasons why we're so polite to strangers - I'm assuming we all are because we're civilised beings - is because we do not want to end up looking like we were the one raised up in a zoo without any proper training in social behaviour. Surprisingly (or should I say not?), when we've known someone for a substantial period of time, that feeling of having respect and showing some courtesy towards the said person disappears. It's true. Try doing a cohort study on it. Get to know somebody new in your life. Note how you treat the person during the first couple of days/weeks/months and continue this practice for a reasonable amount of time. The outcome of this can be anything from ending up as husband-and-wife to staying merely as acquaintances, but the important thing is to note how you behave towards that person.
Do you still stop yourself short of saying 'Yes, your bum does look big in that outfit'
Are you starting to roll your eyes whenever it's their turn to talk?
Do you find yourself starting to act like a self righteous bastard with the person?
I'm not saying that I don't do it - I do. Especially the bastard bit, although I wouldn't really call myself self righteous. Ask my parents. They are sure to tell you of countless occasions where they took heart with what I said. Ask my friends. I'm sure there have been comments in the past which they have never really forgiven me for it. Ask anyone who knows me - you'll get the same kind of answer.
As cliched as it may sound - don't take people for granted. Unless you happen to be a baboon with inept social skills (which I am sure you are not), always remember that the people you meet in life may not necessarily always be there for you. And when that day comes, will they leave with fond memories or otherwise?
Note to self: why so serious?