There are numerous reasons why a given person will tell a falsehood, such as to protect themselves, or to protect another person, or to not be found wanting, or to not be held accountable and so on. The thing is that lying has a way of coming back to haunt us, because when people feel that what we are saying is rather suspect, it hurts our credibility with them. Basically, when a liar has been found out, people have a tendency not to believe what they are saying, because they don't trust them and because of that lack of trust, they don't believe them, even when they actually are telling the truth.
Look, it has to be said, people of real integrity don't tell lies, or else they wouldn't be people of integrity and for all those that wish then to reach that high standard, they are going to have to make up their mind that telling lies must not be part and parcel of their makeup. Those then that determine that they will be people of integrity, must be on guard to be that very thing, all of the time, for if there is a chink in their armor, this then will reflect that they haven't obtained what they need to have obtained, so as to be that person of integrity.
The reputation that we develop may or may not be totally fair, but it is probably quite fair to say, that those that have a reputation of untruthfulness and those that have a reputation for truthfulness are probably correctly categorized, for people can only be fooled for so long, before they begin to figure out that not all is right with those that are actually being deceptive to them; whereas, for those that are honest, such will mesh up well with the crosscheck so made by people to verify their veracity, and those that are consistently true are thus categorized as being honest with us.
So too, the problem with lying to people, is that it is hard to keep track of every single lie and who that lie was spoken to, for people will lie one way to one person, and another way to another person, making it difficult to keep it straight in their own mind, exactly what they said and to whom. On the other hand, those that tell the truth, don't need to worry about keeping their story straight, for their story is always the same, subject to honest mistakes, like forgetfulness.
So then, if you want people to actually believe what you are saying, the surest pathway to accomplish that goal is to be a person of integrity, and even if previously you were not such an honest person, the bottom line is that your reputation will eventually catch up to who and what you presently are. As for those that don't care so much about being honest, they must surely recognize, that their dishonesty will often by met by other people that will be dishonest to them, for like attracts like.