To challenge himself? To try to become the greatest?
When Anderson was fighting Mach Sakurai he didn't look like a guy that could dominate at 205.
There are lots of challenge's for GSP at 170.
Not counting the rematches there's a whole new lot of top contenders knocking at the door for a title shot (Maia, Kampman, Hendricks, Rory, Nick, Erick, Marquart, Ellenberger, Santiago, just to name a few).
Besides, there's the challenge of beating Anderson Silva winning streak and that sir, would be a huge feat in itself.
I'm totally against GSP moving up or down, he's perfect for WW. I would however embrace a super fight versus Anderson for the WW title if
Andy could make 170.
McCall, Faber, Aldo, Nate, Nick, Lyoto, TBD, Hunt
Forever hanging in Anderson Silva nuts