Because we are considered polite only when we say what we are expected to say or when we stay silent. Parents have always told us not to lie, but at the same time think what we say. Have you noticed that little kids can't really lie? That's because they haven't been in the social situations where lying is the only way. When kids go to school/ kinder garden, they start lying (to their parents, classmates, teachers). Sometimes those lies don't have consequences, but sometimes they do. That's why our parents told us not to lie. But telling the truth also might have consequences. That's why sometimes it's easier to say what we are expected to say. I suppose all adults lie more than once every day, though they say it's not okay to lie. You can lie to your mother about your health issues, so she doesn't worry, and you think it's better that way. Maybe it's so. In conclusion, we lie when we realize that it's more convenient that way. That's how our society works.