"...I can't speak for everyone else, but how often do you think about what you believe about your eating a sandwich? Assuming your kind still eats regular food."

You drink blood to survive. You try to not take more than the vessel's system can take so that you'll be able to drink from them again at some later point. It's not quite as simple as that now that she's been talking to people again for over two weeks, but that's still the gist of it.