Although other animals cannot necessarily be expected to uphold ethical standards, this is not true for human beings. Since animal products are both unnecessary for us and harmful to animals, and since there is widespread agreement that inflicting unnecessary harm is wrong, we have a moral obligation to be vegan.
A more specific answer to this question is that we are not physiologically designed to eat other animals. Amongst other differences, carnivores have sharp teeth that can bite through skin or strong muscles to slaughter their prey and eat them whole, entrails and all. Fresh blood and raw flesh make them salivate and kill to satisfy their hunger. When we humans contemplate things such as fresh blood, raw flesh and entrails, there is a sickening guttural reaction.
Imagine catching a deer without weapons (no guns, arrows, spears, knives, or similar items – just your flat teeth, dull nails, and hands) and then eating this animal whole, fresh and raw. Does that sound or feel natural to you?
Unfortunately we do live in a world where some animals prey on others. While these animals don’t have a choice (they must either kill or starve to death), we should be grateful that we do have a choice as to whether to participate in these acts of violence; acts which are, for us, entirely unnecessary.
(Shared with kind permission of Angel Flinn)