Admittedly, Cas could have read her deeper had he desired the uncomfortable headache that came along with using what remained of his Grace. His trusting her was not simply a gesture of good will - like her, self-preservation was something of a concern, though barely. Cas had never cared much for his own well-being, but now that he wished to return home to someone, it suddenly seemed important.
She'd not protested drinking from the bags, so he pinched the end of the tube and grabbed the nearest piece of clean, empty glassware. He wasn't certain how much she would need, so he imagined he would need to fill the receptacle. He didn't imagine she would want to drink from him as if it were a straw and he found he didn't want her fangs near him. Whether she intended for the bite to hurt or not...there was a reservation there he suspected he would not feel, were he still an angel.
"I ask that you be patient," he said, flexing his arm and watching as the beaker filled quickly. "If it's all the same...trust is something that must be built over time."
It was not intended to insult her. Merely to explain his point of view. "You're perhaps the first vampire I've encountered who lacks malicious intent. It seems I need to adjust."