Roman Versus Britannia - What's the Difference?

Britannia was racked with intertribal war. The rebels lived like guerrillas and were constantly on the move. They were being hunted, or were hunting, the legions of rome. So why were the silures and ordo ice is so willing to help caracticus and become a lightning rod for rome's fury? Why didn't they just turn him away, since he wasn't from their tribe? And why did they decide to work together? Well, we're forced to make educated guesses, since there's no written history from the brits perspective.