First, becoming vegan has many health benefits. People who are vegan often live longer than those who are not. This is because many animal products are high in cholesterol, saturated fat, sodium, among other things. Red meat is extremely high in cholesterol and saturated fat, and while it does have protein, the negative still outweighs the positive. Milk and other dairy products are also detrimental to health; humans are not baby cows, and are not built to drink milk. Same goes for eggs. While there are some "healthier" meats, they all still have negative effects in some way, and oftentimes there are much healthier plant-based alternatives. Many vegans are not thin and weak, but are just like other people, if not much healthier, because they get all of the necessary nutrients (which can all be found in plants) yet do not eat nearly as much "empty calories," due to the healthy nature of fruits and vegetables. Also, eating meat is harmful to the environment. Over half of all climate changing emissions come from factory farms. Also, factory farming accounts for a huge part of deforestation. Over-fishing is killing the oceans; around the year 2040 much of the wildlife within the oceans will be extinct due to over-fishing. There is also the morality aspect. What differentiates a cow or a pig from a cat or a dog, or even another human? Both are incredibly smart animals that show emotions, similar to humans. What gives humans the right to exploit other animals in this way? Just because humans hypothetically can do this does not mean that they should. The amount of cruelty in the farming industry is horrendous. Of course, the last part is opinion, but the first two parts are not. The negative effects of animal farming on human health and the environment is proven. Sure, there are some negatives to going vegan, like giving up certain foods, but the positive definitely outweighs the negative.
Vegans can be very unhealthy because lack of nutrition in their diet. For egg-xample, they cannot eat dairy. Dairy is full of good vitamins and good fats. In turn, they will be very thin and weak because they only live off of fruits and vegetables. However, it is their choice and no one can stop them.
There are many vitamins and other nutrients that the body needs to function. A dietician will know these and what foods they can be found in. The average person does not. Therefore, saying all people should be vegan is a blanket statement that in practice would limit the nutrients the body receives for most people. For example, many people know there is calcium in milk, but taking that away, how many know there is calcium in spinach?