Since day one, even as "The Prototype," he has always displayed a strong sense of dedication towards making the sport as good as he possibly can, with his contributions and staunch degree of hard work. If I were to ask you to find me even one person on any professional wrestling roster who is as dedicated to the betterment of professional wrestling as John Cena is, I believe you would be hard pressed to come up with that person.
His dedication is evident in his speech, his actions, and his overall demeanor. He loves the business and he is committed to entertaining the common wrestling fan to the highest degree and it shows.
After all, the man has accomplished everything and then some, he has nothing left to prove and it's not like he's hurting financially. So, why not just walk away and live happily ever after, down by the beach in South Florida, in your mansion, with your smoking hot girlfriend?
Because his dedication trumps his materialistic desire to let it go just yet. He still has fuel in the tank and until he can physically go no more, you can expect to see John Cena performing at the highest level possible.