Poor Uriah. He's so excited to leave the city and explore the world, optimistically thinking, "There has to be something else" (7.25), just like a Dickensian urchin finally leaving the orphanage and to find success in the real world.
Despite the fact that the real world isn't all Uriah hoped it would be, he remains himself: flirty and optimistic... at least until a bomb goes off and puts him in a coma that he never wakes up from. Uriah has to die in order to give Tobias the kick in the butt he needs to stop making reckless decisions.
Uriah is pretty much perfect, which makes his death all the more difficult for Tobias to bear. Tobias wouldn't have cared as much if it had been Caleb or Peter who had died, but his best friend, arguably the only happy character in the whole book? That shatters Tobias's world.