People lie. That is the truth of the matter and there are virtually no exceptions to this general rule. While there are all sorts of reasons why people lie, and continue to lie, the most basic reason why people lie, is because they do not desire to own up to the truth of the matter; and thereby their lying is their way to escape punishment as well as fair justice so deserved by their having lied to protect their own self.
The thing about lying is that even when those that lie, know for a certainty that they have been caught in a lie, they have a strong tendency, more times than not, to lie even further to defend and to protect what they were lying about in the first place, by concocting even more lies, whenever they are being directly confronted by someone that knows that they are lying. Behavior like that, is toxic for relationships because obviously when a person has been knowingly caught in a lie, and thereby weaves even more lies in a futile attempt to either deflect from the culpability of the first lie, or to mitigate such, it makes for a distinct form of disrespect and a clear form of someone that represents dishonesty.
So too, when people lie about things that are relatively trivial, or at least not that big of a deal, the only proper conclusion that the offended party can come away with, is that the person so lying about this particular small matter, must thereby feel, for whatever reason, that they cannot willingly admit their lie, perhaps because they believe it reflects poorly on their character or is some source of embarrassment, never seeming to realize that by not testifying to the truth that their character is further tarnished, all the more, by failing to own up to that smallish lie, thereby signifying for a certainty that they surely won't own up to a big one.
Then there are those people, that do sort of, or appear to, own up to their lies, but insist upon re-categorizing such as not so much a lie, but rather as something more akin to an error in judgment or an inadvertent mistake and thereupon come up with all sorts of reasons, excuses, and basically lies, to concoct a story that subsequently changes the facts or the circumstances of such, that it thereupon appears to be not so much a lie, but just some sort of misunderstanding. The problem with this, though, is that those that actually make an error or have made a mistake, and have such honestly and without judgment pointed out to them, usually have little hesitation in owning up to such, and almost always thereby sincerely apologize to the other person for that error or mistake; whereas someone that is liar, doesn't bother to apologize for such, at all.
It wouldn't be so bad, if people that lied, did so primarily because they desired so strongly to be seen always as a paragon of virtue, but rather the fact is, those that lie the most, and lie at those times when to admit truth would subject them to some sort of punishment or admonishment, are people of weak character; that typically also insist that the other party always be honest with them, while they on the other hand, are conveniently dishonest when it so suits them. The thing about habitual liars is that besides the fact that they lie, is that at the end of the day, they do not want to put forth the effort so needed to change their heart, but would rather prefer to lie about such a change of heart, instead.