Sometimes people hide the truth because some causes. For example, you saw that girls from your class told not really good things about your best friend. In cause you don't want to disappoint your friend, you have to lie and say that they told she is a good person. It can be an example of white lies because you had a good reasons to hide the truth. On the other hand, it's a fact that some people lie because of selfish reasons. Just imagine that boy from your class got a bad grade. But he want to be a good student in the eyes of his parents. He rewrite the bad grade in his school diary on the good grade. He lied both to himself and his parents. It can be an example of dark lies caused by the selfish and mercenary motives.