First of all, humans actually aren't meant to eat animals, our body doesn't even digest meat, it literally rots in out stomach for about a week before it breaks down enough for our system to handle it. The average human body has an extra 7-10 pounds of rotting meat in our systems at any given time.
Secondly, what SmilingKitsune said is correct, killing cows produces a LOT of Methane into our air, the top methane producers are slaughter farms and garbage dumps.
Raising cows is also difficult, they pollute the land win their excess (poop and peepee, kiddies) and if it is not properly cared for, then it seeps through the ground and into water sources. Now, that takes alot of excretion to happen, but thankfully, mass cow farms have more then enough cattle to speed up the process. This isn't a problem, if only the farmers could properly clean up the farmland, but it is just too hard with so many cows and land, so it is often done not to standard, or not at all.
Just a random fact, if every one had 1 day a week that they didn't eat meat products, it would be the same as if around 15 million cars weren't running for that day.
I'm not a vegetarian, so please don't start saying I am, I just eat organically grown food. Less pollution, no chemicals used, and I never liked the fact that my cows were beefed up on steroids anyways.