Hello to you!
You ask a very good question that makes me think :)
In my opinion, there would be several reasons that would make someone lie:
He may want to avoid hurting the other person by telling the truth, so it would be out of a feeling of empathy or to preserve a social relationship.
He is trying to protect himself, so it would be out of a sense of fear.
He is trying to profit from a lie, so it would be out of greed.
These are just my ideas ! I advise you to do research on an Internet platform to have valid sources and read more reliable articles! :) Just by writing "Why are we lying", you should get several interesting results.
Do not hesitate if you have other questions, it will always be a pleasure to answer you! :)