Why do people lie?

Why do people lie? Some said, people lie because they don’t want the truth prevailed. They are hiding for something and don't want the people know the real truth. As a human, we had the right to know the truth specially in a relationship. They said the Truth really HURTS! I rather get hurt and know the truth, than stay cool and think everything is fine but it’s really not. It’s just a nightmare having someone that is a damn liar. What’ is the sense of having a relationship when someone is not real and u cannot trust at all. Why can’t just tell the truth so it would be finish. Why pretend that everything is fine… Always remember “You can’t keep smoke in a bottle” and Truth will always prevail maybe not now but later…