The why is a bit more complicated and diverse, but the core goal is always because of the subject wanting something and deception is often a straight forward tool to acquire it – including pathological and compulsive liars.
Now to the parts of a deception. Unlike the (raw) truth where there is only 1 part to it, (all) lies have 2 parts; the creation of the untrue information and the intentional suppression of the true information.
Deception is indeed a survival-instinct but to the vast majority of people, it still feels unnatural and forced. This along with the strain of additional mental processing causes the body to react abnormally and involuntarily.
These are the indicators of deception. Most are hardly noticeable unless you already know what to look for and especially after determining the subject’s truth baseline.