What we must do is bring nonhuman animals within our sphere of moral concern and cease to treat their lives as expendable for whatever trivial purposes we may have.

I've been reading Peter Singer's Animal Liberation this morning, and I wanted to link to part of it, because it's very good, and because it's a very well-considered and persuasive answer to the question of why a person ought to be a vegetarian (not that I'm expecting any sort of mass conversion here -- some of my best friends eat hot dogs). Here's one version of the book's opening chapter, "All Animals Are Equal." Here's another.

Here's the nutshell to get you started:

If the case for animal equality is sound, what follows from it? It does not follow, of course, that animals ought to have all of the rights that we think humans ought to have - including, for instance, the right to vote. It is equality of consideration of interests, not equality of rights, that the case for animal equality seeks to establish. But what exactly does this mean, in practical terms? It needs to be spelled out a little.

If I give a horse a hard slap across its rump with my open hand, the horse may start, but presumably feels little pain. Its skin is thick enough to protect it against a mere slap. If I slap a baby in the same way, however, the baby will cry and presumably does feel pain, for its skin is more sensitive. So it is worse to slap a baby than a horse, if both slaps are administered with equal force. But there must be some kind of blow - I don't know exactly what it would be, but perhaps a blow with a heavy stick - that would cause the horse as much pain as we cause a baby by slapping it with our hand. That is what I mean by the same amount of pain; and if we consider it wrong to inflict that much pain on a baby for no good reason then we must, unless we are speciesists, consider it equally wrong to inflict the same amount of pain on a horse for no good reason.