Beacuse "there is a truth behind every lie"..

Because it is part of our nature to lie since humans are egocentric and even without noticing we try to look good before other and have their approval. Also we believe lying we can get things easier.

People lie because they either want to get something or afraid of losing something. One may lie because he/she doesn't want to lose face, get a job or don't want to lose a job, to get attention or respect or not to lose someone ( a lover or friend) or even get someone (a lover or friend). In my humble opinion, it all boils down to this, gain/get something or afraid of losing(keeping something already exists). But again why lie to get or lose something? because people are not perfect and people don't want to accept that, so they lie to appear as if they are perfect or almost perfect.  

There could me many reasons to lie for some one. Here are few of the possible reasons:

1) To cheat.

2) To harm or hurt others.

3) To hide their own faults/shortcomings/mistakes/blunders/crimes etc.

4)To gain undue advantage in any situation.

5) To gain money or something by unfair means.

6) But I think the biggest possible reason is lack of morality & values, lack of self belief, and also lack of belief in natural justice.

I think there are many reasons why people lie.

One reason can be what M.Eslamouly said. Another reason could be is that they want to protect someone else from harm, physical or emotional. Another thing I could think of is that they want to cause trouble, so they'll make something up and put it against someone.