He has nothing to prove? What? Of course he has tons to prove. So far all he has proven is that currently he is the best in the UFC.
In Pride Chucks record is 2-1.
Best fighter Chuck has faced yet? Randy 3 times. Chuck was 2-1 vs Randy.
Chuck vs Jeremy Horn is 1-1
Chuck vs Rampage is 0-1
Chuck has beaten other "good" fighters but other than the ones listed above has not faced any "great" fighters. Chuck's record vs these great fighters is 3-3.
Some of the "good" but not "great" fighters he has faced is:
As you can see Chuck vs the "great" fighters has a 3-3 record, Chuck vs the "good" fighters has been 7-0, Chuck vs "average" fighters has been 10-0. I person could argue to juggle the names on my lists a little but the point is still valid.
So to finish Chuck is a very very good fighter, but still with lots to prove vs the "great" fighters. There is still many fighters out there that are "great" that Chuck has not faced. Chuck is the best LHW currently in UFC, but not in the world.