Why was Rufus angry when Mr. Rubio mentions football?

Rufus is angry because he believes that his privacy has been invaded.

Mr. Rubio kept peering at him. Rufus's expression did not change, but inside he was angry. Baby Gibson, who had gone home a few weeks ago, had obviously told someone about Rufus's scrapbook. Rufus had over a hundred pictures of Ernie Brown, the Cleveland Corsairs' great halfback, in the book. He had collected them for years and bound them into a big loose-leaf volume with a picture of Big Ernie on the cover. Baby was the only person at camp besides Rufus who knew about his Big Ernie book. He had left it at home when he was sent to camp, knowing the headshrinkers up here would ask so many questions about it that he would have to bum the book to turn them off.