"Because - " Kitty bites her lip. There aren't really good reasons - at least, not from his perspective. She and Bartimaeus aren't friends. Not really. Back home, they might have had a common interest - ridding themselves of the rule of magicians - but that's not a factor here. And they don't really owe anything to each other. Or - Well -

"Because I owe you." She shoves her hair behind her ear, feeling formlessly anxious. "You saved my life back there, didn't you, and I haven't properly repaid you."