Explain to me why people decide to go vegan.

Vegans are primarily concerned with animal welfare. Humans do not need to eat animal products to be healthy, so using animals for food is unnecessary. Under capitalism, trillions of animals are raised in horrible conditions each year to be slaughtered to satisfy human taste buds. However, even if an animal is raised in conditions better than a factory farm, killing them is purely for taste since humans do not need to eat them to survive. Vegans view this as immoral and choose to not use animal products. 

Additionally, meat has a high carbon footprint compared to eating plants. Beef is a particularly bad offender. Vast swathes of the rainforest are cleared away to make room to raise cows for slaughter. Many vegans also care about the environment and avoiding animal products is a way to contribute less damage to the environment.