People lie on others to hurt them or take advantage of them. Keeping someone in a defensive posture keeps them from moving forward. They can't see the big picture because they're too busy trying to disprove things.
One example might be your mate accuses of cheating because they're cheating. You become consumed with trying to prove your innocence that you ignore the "red flags" which indicate they're the one who is cheating.
In other instances there are people who BLAME others for (anything) that goes wrong/hurts or is missing in their lives!
Last but not least there are some people who are paranoid and insecure by nature. They don't love themselves and therefore cannot believe anyone else could love them either! They may have some mommy/daddy issues where they were mistreated or felt abandoned. From then own they decided (never) to trust anyone.
"Never love anyone who treats you like you're ordinary."
- Oscar Wilde
If someone accuses you of things and sees you as liar or (untrustworthy) they clearly don't think you're all that "special".
It's important to remember people don't "change" unless (they) are unhappy with the results they're getting. It's usually a mistake and a waste of time to stick around someone who insists on having you "walk on eggshells" whenever you're around them or have to prove your love over and over again. You're better off finding a person who is "emotionally healthy"! Avoid being a codependent.