Why do people lies lie? 

To speak the absolute truth is hard for most of people.
Why are do some secrets remain concealed?
Sometimes we should have lied lie not (in order) to not offend our friends and relatives, should not shouldn't we? (or "should we not?")
It will be lie but it has a good goal. (It will be lying but for a good cause.)
Probably, in the modern world, there is not any person who would does not lie (at least) once a day or week through. (Ok, but I would say "In the modern world, there is probably no person who does not lie at least once a week.")
Why do people resort to lie lying, rather than telling the to truth? (or "Why do people resort to lying instead of telling the truth?")

Well, like you said, speaking the absolute truth is hard for most people. They could be afraid of being judged or punished, whatever their reasons may be. Lies told for a good cause are called "white lies". Sometimes, the absolute truth may not be all that matters. :)