Why do people hate on other peoples work?

I detail cars with my freind and I have been doing it since I was 12 and now Iam 16 and I know what Iam doing and if I dont know what Iam doing I DONT DO IT. Well I have lots of happy customers and they refer me to other people and so on and on. And I have been doing my freinds Subrban for a while and Iam giving him a killer rate and he knows it and he loves my work. Well hes trying to sell it and the kid he might be selling it to wants it detailed if he buys it, and the kid that wants to buy it knows me but has never seen my work when its clean only when its been rained on. Well the kid keeps saying I do shity work dont take it to me hes awful and sucks and he dose it ghetto and then when I talk to the kid he acts like he dosent say that. But why do people talk shit about someones work when they have never seen it or dont know what they are talking about.

I mean its really fucking gay that he talks shit about my work when he has never seen it.