Christine M. Korsgaard was kind enough to answer a few questions of mine. Here's an excerpt:

ERICH: Many animal welfare advocates seem to be utilitarians, possibly due to the influence of Peter Singer. But you, of course, are not a utilitarian. Why is it not a convincing moral philosophy, in your view?

CHRISTINE: Because I believe that everything that is good must be good-for someone, some creature, I don’t believe it makes sense to aggregate goods across the boundaries between creatures. Of course, if you say “I can do something that’s good for Jack, or I can do something that’s good for Jack and also good for Jill,” everyone thinks that the second option is better, and that makes it look as if aggregation makes sense – the more good, the better. The problem only shows up when you have to do some subtracting in order to maximize the total. If Jack would get more pleasure from owning Jill’s convertible than Jill does, the utilitarian thinks you should take the car away from Jill and give it to Jack. I don’t think that makes things better for everyone. I think it makes it better for Jack and worse for Jill, and that’s all. It doesn’t make it better on the whole.

Of course, behind this there is a deeper problem. Utilitarians think that the value of people and animals derives from the value of the states they are capable of – pleasure and pain, satisfaction and frustration. In fact, in a way it is worse: In utilitarianism, people and animals don’t really matter at all; they are just the place where the valuable things happen. That’s why the boundaries between them do not matter. Kantians think that the value of the states derives from the value of the people and animals. In a Kantian theory, your pleasures and pains matter because you matter, you are an “end in yourself” and your pains and pleasures matter to you.