Though I wasn't particularly aware of this question turning in my head, it must have been doing so for some time.
Pressed every once and a while (I'm not the sort of person people press often) to explain why I don't eat meat, I usually respond:
1. We don't need to eat meat.
"One farmer says to me, "You cannot live on vegetable food solely, for it furnishes nothing to make bones with"; and so he religiously devotes a part of his day to supplying his system with the raw material of bones; walking all the while he talks behind his oxen, which, with vegetable-made bones, jerk him and his lumbering plow along in spite of every obstacle. Some things are really necessaries of life in some circles, the most helpless and diseased, which in others are luxuries merely, and in others still are entirely unknown. "
2. In any case, we American eat far too much of it. (The average American eat nearly half a pound of meat a day!)
3. It is deleterious of our health; of this there can simply be no doubt. I distrust all of modern nutrition, but I can point to the indubitable fact that every culture that has gone from eating a vegetable, grain, oil and fish diet to a diet that resembles our own American "diet" has grown steadily less healthy, more obese, and stupider.
4. Furthermore, you can have a varied--even more interesting--diet without meat."
In fact, I really like meat. You name it, I like it. Even offal and liver and sweetmeats. Beef. Chicken. Birds of all sorts. Horses (yes). Lamb. Wild beast. Pork. I used to eat--and occasionally, I still do--all of this with quite a lot of relish. And sometimes ketchup.
Notice that I have no compunctions about eating meat on the usual moral grounds. I like animals well enough, but I don't think they have souls and I have no guilt issues involved in their slaughter, so long as it is done well and as painlessly as possible. (That said, I don't like the idea of anything suffering needlessly, and so in particular I avoid meat the provenance of which I do not know.) In fact, if you know my story, you'll know that I have slaughtered plenty of animals, and I don't feel any guilt whatsoever.
I am an avid fisherman and really love to eat any fish. That said, I mostly only eat the fish that I catch. Our seas have been roundly abused, and I want as little part in it as possible. Moreover, I will not eat frozen fish of any kind, nor farmed fish--they taste like shit, and life is too short to eat shit. (I will eat frozen squid!) I am fortunate to live in Florida, a state that has some of the best, most progressive, laws around regarding commercial fishing. I only eat locally sourced fish that I know is taken from grounds that are not overfished. (Like I said, I don't eat much fish!) I imagine if I did not live in Florida, I would not eat fish at all.
In the end, for me the most compelling reason not to eat meat, or to eat much less of it, is the incredible waste involved. If you do any reading, you will be astonished by the amount of water, grain, labor, oil, fertilizer, concrete, wood, wire, electricity, herbicides, pesticides, antibiotics, legislation, oversight and carbon dioxide involved in manufacturing (and that's the correct word) a pound of meat.
Four-hundred gallons of water, lord knows how much fossil fuel (but lots of it), seven pounds of corn and all the concomitant fertilizer/herbicide/pesticide... to produce a pound of beef, much of which goes into eating that shitty shit people wolf down in fast-food restaurant. Shameful.
And then you have to deal with the enormous impact of fantastic amounts of shit. A hundred-fifty pounds of shit A DAY for a cow!
(Many will rightly point out that grass-fed, pastured cattle are far less resource-intensive, which is true. But the amount of "free-range" beef produced out there is a fraction of what's consumed.)
Yes, resources are used to produce a pound of vegetables and grain, but a small fraction compared to meat of any sort. And a pound of veg and grain is far tastier and more varied than your chunk of beef, and probably lots cheaper. (I try not to think of food as nutrition (for reasons too complex to explain here), but, there can be no doubt, that pile of veg, fruit and grain is of far greater healthfulness than the beef.)
The world is getting smaller, flatter, hotter, drier, more unpredictable.
Choosing not to eat meat makes sense.
Plus, I have a kick-ass garden.