GSP just has OCD, he's a perfectionist who needs every little thing to be just right otherwise it isn't right in his mind. Everything he does, he does it "the right way". That's why he doesn't want to jump up and down in weight, but he would be willing to go up to MW if it was done "the right way" by packing on muscle and permanently changing weight classes.
He is a very safe fighter and person, and quite simply it would be out of character for him to jump up in weight to fight Anderson. I do believe that he would 100% fight him at 170, though, but I dont think Silva is capable of making that cut.