Why are so many athletes greedy?

I understand that they have unique talents and have dedicated their lives to what they do...but why do they get paid millions to play a game? While people who help others and sacrifice their lives such as doctors, firefighters and people in the armed forces get paid SIGNIFICANTLY less. I mean for example Darrelle Revis turned down tens of millions of dollars because he thinks he deserves to be the highest paid player at his position, like really? What can you do with 17 million that you can't do 14 million?