The human body is like a vehicle which needs to be fuelled. Your body needs to be energized through having specific portions of various foods. Throughout your life, your physiological system got used to a certain dietary system, especially if you stick to one diet for a long time.
This means that any slight change in your diet is going to turn everything upside down in your body. Vegetarian and vegan people were not born as such, but they experienced few changes in their lives which made decide to become so.
Throughout their journey to become vegans or vegetarians, they went through a lot of adjusting levels that to adapt to the new dietary system they drew. If you strike one thing out of your diet, you need to replace it by something which will provide your body with the needed and missed nutrients.