We have lots of reasons for telling lies - avoiding hurt feelings, promoting ourselves, avoiding embarrassment, escaping punishment, selling band instruments...etc.
Donald Trump exhibits an interesting, perhaps less obvious, reason for telling lies.
He uses lies in a power game to demean and dominate those around him.
Here's how it works...
He says something he knows is a lie and the people around him know is a lie.
If the people around him don't call out his lie he has successfully exhibited his dominance over them.
The more harmful the lie the more demeaning it is for those unable or unwilling to call it out - hence the greater the dominance the liar exhibits over those accepting the lie. To put it a little differently...
The size of the piece of our soul that we sell by not calling out the lie is proportional to the potential harm of the lie. Ignoring a lie with a greater potential for harm requires we give up a greater portion of our soul (our self). We demean ourselves more (make our selves less human) as the lies we refuse to call out increase in their ability to cause harm.
It's simple and pathological. Anyone who has worked in a hierarchical organization is probably familiar to some degree with this method of exhibiting dominance.
This inability or unwillingness to question power and authority, being willing to go along with the lies, has led to tragic consequences throughout history...from wars, genocides, poorly designed products that kill people, to the ultimate tragedy of destroying our only home by going along with the biggest lies of them all...
Having been entrusted with caring for a planet with limited resources and multiple sensitive ecological systems, rather than conserving that gift for future generations, we accepted the biggest lies of all - that unlimited consumption, unlimited economic growth and unlimited use of fossil fuels was not a selfish fantasy that is destroying our planet, but our patriotic duty.
I started thinking about this idea of lying as part of a dominant/submissive power game when I heard Mary Trump (Donald's niece) in an interview describe meeting one of Donald's wives for the first time. In introducing Mary to this wife, Donald says Mary was a drug addict. Mary and Donald both know this is a lie. Donald has demeaned Mary first in calling her a drug addict and second by bullying/daring/dominating her into not calling out his lie.
"In many ways the performances of Donald Trump remind me of male chimpanzees and their dominance rituals. In order to impress rivals, males seeking to rise in the dominance hierarchy perform spectacular displays: stamping, slapping the ground, dragging branches, throwing rocks. The more vigorous and imaginative the display, the faster the individual is likely to rise in the hierarchy, and the longer he is likely to maintain that position.”
He's a bad monkey but an even worse man...somewhere inside his head he knows that. His whole life consists of multiple unsuccessful efforts to forget who he really is. He's not alone in that respect just a particularly glaring example.