Why does Theo need to challenge the warden's authority?

There are a number of reasons. Though Xan—the Warden of England—and Theo are cousins, they're not very close. In fact, no one is really close to Xan, the self-contained, mistrustful, and isolated dictator. As a professional historian, Theo is naturally concerned that the continuing infertility crisis will effectively mean an end to history. His family connection to the Warden of England, as well as his growing disillusionment, make Theo an ideal recruit to the Five Fishes, an anarchist rebel group. It is while on the run with the group that Theo falls in love with Julian, one of the rebels' leading members. His growing emotional attachment to Julian provides Theo with a need to challenge the Warden's authority, not least because the Warden himself is also in love with Julian and wants to start a new race of people with her.