Religion: Some religions encourage their followers to abstain from meat or dairy products, or both.
Environmental: Livestock and animal farming produce a lot of greenhouse gases, so people may reduce their intake of animal products to lessen the negative impact on the environment.
Ethical: They do not want to contribute to exploitation and cruelty in the farming industry by buying, or consuming, meat or dairy products.
Health: Some people lactose intolerant, others have allergies or intolerances to various animal products, and some may have diseases or disorders that make consuming animal products unpleasant or dangerous.