Why does Timothy's voice sound anxious when Phillip asks him to promise to never leave him the cay?

Timothy's voice sounds anxious when Phillip asks him to promise never to leave him on the cay because he understands the gravity of their situation. He knows that survival is uncertain and that circumstances can change rapidly, making promises difficult to keep. His anxiety reflects his deep concern for Phillip’s safety and the reality of their isolation, highlighting the bond they share and the weight of the responsibility he feels for Phillip’s well-being.