I suppose I had my rock star fantasies while I was singing into my hairbrush in the bathroom mirror, but I never really consciously said, 'OK, this is what I'm going to do for a living and I'm going to be Weird Al.'
So that's why one of my rules of parody writing is that it's gotta be funny regardless of whether you know the source material. It has to work on its own merit.
It's hard to really articulate what the parameters are that make one song parody-able and another song not, but if I can come up with a good enough idea for it, I go for it, and if not, then I have to move on.
At this point I've got a bit of a track record. So people realize that when 'Weird Al' wants to go parody, it's not meant to make them look bad... it's meant to be a tribute.
In fact, when I come up with an idea for a parody I try to resist the urge to Google the idea to see if someone has done it already because the answer is almost always, "Yes, of course they have, they've thought of it!"