I've been vegetarian for nearly 4 years and now starting to move towards being vegan! I already have b12 injections because of being deficient in it though - but that wasn't because of my diet.
My reasons are mostly environmental - livestock agriculture is one of the main contributors to greenhouse gas emissions, ocean dead zones, species extinction, rainforest deforestation, etc etc. Giving up beef and dairy has a bigger impact on the environment than giving up using a car! Plus if everyone gave up meat, we would easily solve food and water shortages, as the grain and water that are used to feed livestock could be given to humans.