Personal: Some people believe it is wrong to kill and eat animals. Some simply do not like the taste of meat.
Religion: Some religions do not permit their followers to eat meat (for example, Muslims are not allowed to eat pork).
Health: Some people choose to become a vegetarian for health reasons because meat is high in saturated fat.
Environment: Some people believe that meat production pollutes the environment more than crop production.
Social: Some people are influenced by their friends and family. If parents or siblings are vegetarians, people can be more likely to become vegetarians too.
Economic: Meat is often more expensive than plant-based foods.