Why does my friend have to lie?

so iv got this friend who claims to have slept with many many women.it seemed a bit corny when he first told me upon meeting of his tales of bedding strippers and 3somes with hot groupies from his band but i believed him at first.he would always pursuade to me come with him to go meet "lots of hot chicks" he knows who he claims to have slept with. but the majority of the time, these girls either never showed up or acted completely uninterested in him. many i became friends with said that nothing ever happened.one of these girls eventually became my girlfriend and as me and her grow closer he gradually changed his story hoping i wouldnt notice.even after me and her split up he continued to tell these stories.many people will make fun of his stories or sarcastic remarks "oh of course,what girl would resist you".now he has a girlfriend and hes stopped telling these stories saying they were "silly mistakes from the past".iv not said anything incase his girlfriend doesnt know anything about these or i embarass him.what are the main reasons for guys lying about sexual exploits and why does he feel the need to lie to me?