Lots if reasons. Sometimes they don't like the truth. Sometimes the truth hurts them or will the people they're telling it to so they lie. Sometimes they do it to get away with stuff. There are a lot of reasons that people lie.
Most lies are things we say so we don't have an argument or have to discuss every single thing that can come up in a conversation. If somebody asks how they look in an outfit, you might say OK even if you don't personally like it. Have you ever said Yes, when your parent asked if you finished your homework? It gets bad when you lie about obligations or become one of those people who lie even when they don't have to.