Vegan because it is wrong to harm and kill animals when we don't need it for health

 Veganism rests on:
"Although there is a great deal of disagreement about moral issues, no one disagrees with the notion that it’s wrong to inflict unnecessary suffering or death on a human or an animal. We need a good reason to inflict suffering or death on a human or animal. We might disagree about whether necessity exists in any given situation and what constitutes a good reason, but we would all agree that enjoyment or pleasure cannot constitute necessity or serve as a good reason. This is part of our conventional moral wisdom.

Consider an example from the human context. If a person said that she believed that it was morally wrong to inflict unnecessary suffering on children but that beating children for pleasure was morally acceptable, we would understandably be confused. If enjoyment can suffice as a good reason to beat children, then there’s no bad reason to beat children. The principle that it’s wrong to inflict unnecessary suffering on children would be meaningless."

This is something that almost everyone agrees with.
If you believe in this, and want to live according to your beliefs, you will understand it is a moral obligation to go vegan: http://www.abolitionistapproach.com/what-michael-vick-taught-us/#.UfDiaW34V5s

Health:
“It is the position of the American Dietetic Association that appropriately planned vegetarian diets, including total vegetarian or vegan diets, are healthful, nutritionally adequate, and may provide health benefits in the prevention and treatment of certain diseases. Well-planned vegetarian diets are appropriate for individuals during all stages of the life cycle, including pregnancy, lactation, infancy, childhood, and adolescence, and for athletes.” — Academy of Nutrition and Dietetics (formerly the American Dietetic Association)

B-12: http://www.tcolincampbell.org/courses-resources/article/b12-breakthrough-missing-nutrient-found-in-plants/
Vegans who eat an adequate diet, and possible get their B-12 from e.g. supplemented soy milk/grow their own vegetables in soil with B-12, eat a diet that they thrive on and are healthy on.

A diet of a big amount of animal products is detrimental for human health: http://www.adelicatebalance.com.au/