When you eat dead animals, you are consuming the decomposing corpse of a creature who died in fear and pain, so there is an argument that you take in those stress hormones. You also consume fat that's saturated at body temperature and will hang around in your arteries, to fur them up. (With a couple of exceptions, like palm and coconut oil, vegetable fats are liquid).
You also have bits of decomposing corpse passing through your intestines for several days. Try mashing some meat up, splashing it with vinegar or lemon juice to replicate stomach acids, putting it in a warm place for a few days and then taking a look and smell.
You can get all the nutrients you need from plants, without the hormones, puss, cholesterol and other nasty stuff you get in meat and dairy products.
There are three reasons for eating meat:
1. Tradition and not wanting to seem different from your friends and family
2. Lack of imagination
3. Liking the taste too much to consider alternatives
...but millions of reasons to not eat meat, such as:
* Better health and energy levels (but your diet must be balanced; there are unhealthy vegans too)
* Concern about all the food poisoning bugs that thrive in both close-crowded animals and their decomposing corpses and the ineffectiveness of all the antibiotics used to contain them. (Fish are the biggest risk as the bacteria that live in them are adapted to cold temperatures like fridges)
* Opposition to animal slavery (the rearing and killing of one set of beings purely to serve the purposes of another)
* Opposition to the boredom, frustration, fear and confinement of farmed animals, plus the pain and terror they experience when killed
* Opposition to humans starving around the world while good quality protein is fed to livestock
* Opposition to the vast amounts of water, air and soil pollution caused by livestock
* Opposition to the huge amount of electricity, water, land and other resources that go into producing, processing and storing even a tiny quantity of meat
* Opposition to the collective insanity and apathy that makes this kind of food "normal" while a plant-based diet is still regarded as cranky
...if these reasons aren't enough, there are a hundred thousand living, breathing, bleating, snorting, clucking ones per new vegetarian who don't have names, but who would make themselves known to you and thank you if they could.