Why do Peeta Mellark and Katniss play again in Catching Fire?

In the book "Catching Fire," Peeta Mellark and Katniss Everdeen are forced to participate in the third Quarter Quell, a special edition of the Hunger Games that commemorates a past rebellion. They do not voluntarily choose to take part in these games again but are compelled to fight in a life-or-death competition once more due to political circumstances and the Capitol's rules governing this special event.