It's really not rocket science. If animals are not mere things; if they have moral value, we cannot justify eating, wearing, or using them particularly when we have no better reason than palate pleasure or fashion. If you are eating, wearing, or using animals, then your actions say that you regard them as mere things, despite what your words say.

We can no more justify using nonhumans as human resources than we can justify human slavery. Animal use and slavery have at least one important point in common: both institutions treat sentient beings exclusively as resources of others. That cannot be justified with respect to humans; it cannot be justified with respect to nonhumans—however “humanely” we treat them.

Ethical veganism results in a profound revolution within the individual; a complete rejection of the paradigm of oppression and violence that she has been taught from childhood to accept as the natural order. It changes her life and the lives of those with whom she shares this vision of nonviolence. Ethical veganism is anything but passive; on the contrary, it is the active refusal to cooperate with injustice

We proclaim human intelligence to be morally valuable per se because we are human. If we were birds, we would proclaim the ability to fly as morally valuable per se. If we were fish, we would proclaim the ability to live underwater as morally valuable per se. But apart from our obviously self-interested proclamations, there is nothing morally valuable per se about human intelligence.

Veganism is not about giving anything up or losing anything; it is about gaining the peace within yourself that comes from embracing nonviolence and refusing to participate in the exploitation of the vulnerable

...eating animals involves an intentional decision to participate in the suffering and death of nonhumans where there is no plausible moral justification.

99% of our uses of animals, including our numerically most significant use of them for food, do not involve any sort of necessity or any real conflict between human and nonhuman interests. If animals matter morally at all, then, even without accepting a theory of animal rights, those uses of animals cannot be morally justified.