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.
Lyoto Machida, former UFC LHW hamp
Future UFC MW Champ
Benavidez, Barao, Aldo, Nate, Nick, Lyoto, TBD, JDS
Forever hanging in Anderson Silva nuts