This article claims that masters continue to compete because it's who they are. Nothing more.
'In the 1973 film "The Paper Chase," first-year law student James T. Hart struggles all year in his contracts class, then folds his unread report card into a paper airplane and sails it into the ocean.
Records and race victories are like that report card. And injuries and slower times are a small price to pay for the chance to toe the start line with our peers, awaiting the starter's command to continue the good race we've been running all our lives.' http://runningtimes.com/Article.aspx?ArticleID=24440