Educate yourself. Easy to do - watch the top 3 films and you will gain the knowledge of why vegan makes sense. Watch Forks Over Knives, Cowspiracy, and the documentary Earthlings (which Joaquin Phoenix calls the film he is most proud of).
This is exactly why I became vegan. Except at first it was for my health, to eat healthy food. And I figured eating vegan was best. And then I started watching the videos like Earthlings and educated myself. I've been vegan for just over 2 months now and