Usually guys saying these are great former champions though. Cain got KO'd in a minute by the same guy in his first defense, nothing about that belt is his to claim.
He wants to be champion again... he considers himself the best im sure. So he calls it his belt because he believes he is the best.
Hell if i was a UFC fighter id start calling the belt mine after my first win. Id probably call the champion out after each fight and tell him to keep my belt warm. Lol