More than 50% of crops and soy we produce is used to feed animals in factory farms. Meanwhile, there are millions of people starving to death around the world.

Animal agriculture is responsible for more than 50% of the deforestation.

Pigs are, by some measures, way smarter than dogs.

About Vegetarianism

Vegetarianism means abstaining from eating meat. It usually applies to the meat of the birds and mammals, the animals most neuroscientists agree are capable of feeling pain. So, why do I adhere to vegetarianism?

Valuing everyone's freedom,
regardless of how diverse
they are,
is a core of many peoples'
ideologies. However,
hardly anyone is consistent
enough to apply that to animals
of other species.
Well, when I started to explore the world using the Internet, one of the first things I found out was that the reasons usually given for eating meat are nothing but modern-day myths.
It's said that eating meat is a natural and healthy way to get the Vitamin B12. However, chances are, the meat you can buy in a supermarket doesn't even contain it. Vitamin B12 is produced by the bacteria in the intestines of some animals. Today, the farmed animals are being regularly fed with antibiotics, which, among other things, kill those bacteria. It's often said that meat is a good source of the Omega-3 fatty acids, which prevent heart disease. Firstly, there isn't much evidence that Omega-3s help prevent heart-disease in humans, and, in fact, there are some good reasons to think they don't. Secondly, animals get the Omega-3 from grass, and most of the animals farmed for meat today never see the grass, they spend their entire lives in the factory farms. The third argument used to justify eating meat is that it's a way to get enough protein. But, here is a thing, we don't actually know how much protein an adult human being actually needs. The figure of 35% of the energy intake written in many biology textbooks is without a doubt a huge overestimation. There are some experts who think that the reason there is so much osteoporosis these days is, in fact, that peoples blood pH is too low because they are taking too much protein. I am not saying they are right, I am just saying there isn't much evidence for some of the well-known things about nutrition.
Some of the other arguments used to justify eating meat is that animal agriculture supposedly helps the environment by fertilising the ground. The fact is, it does exactly the opposite. Actually, it hurts the environment at least as much as the modern traffic does. The other one is that it's supposedly easy to show that animals are incapable of higher cognitive functions. But, what requires little intelligence in humans often requires high intelligence in computers and vice versa, and who knows about animals (see The Moravec's Paradox).
Of course, the most common argument for eating meat is that it's supposedly natural. Now, how exactly people think first humans were hunting before the tools necessary for that were invented, I have no idea. Being able to eat meat probably helped our ancestors survive in some cases. Today, it does more harm than good, and that's clear to everyone who looks at the evidence.
Vitamin B12 deficiency wasn't a problem for our ancestors, because they didn't use to clean the fruits and vegetables before eating them, and many bacteria in dust also produce Vitamin B12. Today, it's probably not a good idea. The best source of the Vitamin B12 today is probably shellfish. Many foods, including some exotic plants, contain it, but our body doesn't absorb it from them well. Since around a third of the worlds' population is deficient in the Vitamin B12, it's probably a good idea to take it in form of vitamin pills. Some plants, like flax, contain a lot of Omega-3. However, it contains a different form of Omega-3 than our body needs, and our bodies aren't good at converting between different forms of Omega-3. So, it's not entirely clear if it's better to get Omega-3 from plants or from fish. The foods richest in proteins are beans and cheese. Of course, not all protein is equal, and what our body actually needs is to get all the essential amino acids from the protein in our diet. The soybeans contain enough of all the essential amino acids except the methionine, and the seeds rich in methionine are sesame seeds. Contrary to popular belief, spinach isn't rich in iron at all. The best vegetarian source of iron is probably lentil. I hope this was enough information for you to understand vegetarianism.
I am not arguing for forcing people to be vegetarian (that would probably just lead to black markets), I just hope I've made some people think. Many people want to make the world a better place. But the right way to do that is to start from yourself. Do you strive not to cause unnecessary suffering of the innocent?

UPDATE on 28/07/2018: I've just written a parody showing how anti-vegetarian propaganda sounds like to vegetarians (and to everybody else, I suppose).