I think the major problem with any form of long-term human relationship is that 'familiarity breeds contempt.'Specifically, the better some people get to know each other, the worse they treat them.
Generally, we're all pretty polite to strangers - but some of us can be outrageously rude to family and friends.
We make an effort to impress people we've just met - but we're bitchy to people we know intimately.
How come people say 'please' and 'thank you' in a coffee shop, but don't dream of saying it over the dinner table?
I think, to be brutally honest, the way people act behind closed doors, amongst the company of their closest family and friends, is how people really are - and sometimes it's not very pretty.
"Rudeness is the weak man's imitation of strength." Eric Hoffer
"Treat everyone with politeness, even those who are rude to you - not because they are nice, but because you are." Author Unknown