I agree with Roger because it's common sense.
When you are young and insanely good, you have an invincibility about you that makes you feel unstoppable. It allows you to hit one of Roddick's nuclear serves back at his feet, but it also allows him to lose to really stupid people from time to time(a few have been mentioned).
Roger's not going to lose to those stupid people anymore. At an older, more mature age, he understands everyone's dangerous, and he's going to prepare for everyone.
That's what makes him better.