A musical interlude

Two people turned up who annoy me greatly. I don't mind that in my professional mode. What annoys me is that every time they see me, they ask me if I can play all sorts of things that I've already told them I can't or won't. The sarky smile tells me they are taking the piss. My usual reply to one of them, a language teacher, is to ask if he can speak all forms of the language, ancient, mediaeval, modern, dialect etc. The other one was being very 'clever' too. He can allegedly sing in two foreign languages, but could not show any evidence of this. I had to help him out with Edith Piaf's La Vie en Rose (Quand il me prend dans ses bras..) and Marlene Dietrich's Where have all the flowers gone? (Sag Mir wo die Blumen sind...). I don't know why he bothered. He then tried to lecture me on popular music, which originated in America at the end of the nineteenth century, according to him. I told him his theory was complete rubbish. I had studied the subject and he had evidently not. I went outside, and this person spent the rest of the evening annoying a friend of mine. Why do some people exaggerate their skills, or claim to have skills they patently don't have? Why do they need to spend their leisure time trying to persuade people of how 'knowledgeable' they are? Why?