I Don't Get It

Why do people ( typically immature guys) always lie about things to make themselves look better?
Like today for example I was this guy I used to talk to and had sex with a while back. And he was getting on my nerves and I told him to leave me alone and he gets mad and wants to lie and say I sucked his dick. Like really? Shit like that pisses me off. And I guarantee you that he's going to ask me later on this week to have sex with him again.😒 
In all honesty he needs to stop worrying about me and worry about his girlfriend that he wants to cheat so bad on with me. And I don't even want him, nor do I want him to think its okay to cheat on someone especially with me.
People  like that just annoy me and I just want reasoning behind his actions. Anyone have any opinions on the situation??🤔