I actually believe that GSP really wanted to retire, and would have done it publicly if his fight ended on a better note with Hendricks.
Now it's a little different, because he came back and won the MW title in a spectacular fashion. It was a really fun fight that was in one of the best cards of all time. It seems like an ideal way to go out on top, and probably would have if it was not for his contract. Now we hear that he has colitis, not doubting him, but with him pushing up in age and what not, it's a good reason for him to hang them up and feel happy that his last fight was a happy memory.