I started after I'd read all of his books for younger people, and had just got 'The Carpet People'. Finding it a bit too young, I rebelled against my Mum saying that the Discworld looked too adult, read the backs of Colour of Magic, and 'Light Fantastic', bought the pair, and also haven't looked back. Other then when I run out and re-read them, of course.
I started becuase, oh, well it'd be more of an explanation of literally 'why'. Because I love Pratchett's way of writing. Under all that light-hearted fantasty, there's a core of black-humoured genius, I feel sometimes.