Let me start by saying that I was raised a vegetarian and have been a vegan for a few years, but what confuses a lot of people is that I'm not really anti-hunting. In fact, I respect those who hunt and kill animals for food far more than the average McDonald's customer. The reason for this is because the hunter at least understands the implications of his meal.
Many pro-meat advocates say that humans are omnivores and eating animals is part of the "circle of life", and I agree with this to an extent. The problem with this is that in the industrial age, we don't raise or prey on animals as much as we manufacture them. I know a lot of (usually female) people who say they'd never kill a chicken or a cow, yet they eat meat regularly. This is because they don't think of their burgers or chicken sandwiches as animals that were killed; instead, they think of them as delicious, cheap food that might have involved an animal at some point, "but it's already dead, so it's best not to think about it."
While it's not practical in modern times to slaughter your evening meal every day, everyone who eats meat should kill for their food at least once. If you can bring yourself to kill for food, then you have a stronger will than I do, and I have (almost) no problem with you eating meat. If you refuse to kill a cow, but demand 1$ Jr. Double Cheeseburgers, you're a hypocrite in denial, and I have no respect for you.
TL;DR: An animal was killed for you to have that sandwich, learn what it means to kill an animal.
EDIT: Just to clarify on a few things:
I said meat eaters should kill an animal at least once. That's my personal opinion; you can do whatever you want, not trying to force you to do anything.
The straw argument of "You should make an x before you use one." is ridiculous. We're talking about a process that involves the voluntary killing of something else, it's a bit different from wood carving/computer assembly ect... As far the "you should grow a plant before you eat one" argument goes: plants lack a nervous system and brain, so it's not really harming a plant to farm one, but for the record, I do have a decent sized vegetable garden in my backyard if it's any consolation.
It is true that every purchasing decision we make has implications we never see. Someone buys a pair of Nike sneakers, and they might have been made at a sweat shop that uses child labor; they just don't know or don't think about it. As a society, we need to learn more about the source of every product we buy, and make well informed purchasing decisions accordingly. I'm just arguing about the meat industry at the moment.
At the very least, you can learn about how animals are killed in factories via internet. It's not pretty, to say the least.
