Because it makes young people sound, like, really unsure of themselves? And, like, really quite thick?
I find it particularly annoying and depressing that teenage girls are the worst culprits: it makes even intelligent young women sound shallow, and quite frankly, inarticulate.
The other depressing thing is that it seems to be infecting much younger children, and most worryingly, even older generations who should know better!
Rant over.