Is There a Justification for Veganism?

I meet a great deal of people, for instance, who are ethical egoists. They say that I just... I don't believe that ethics exists objectively. And so they come to my talks on veganism and they listen and they say, I like what you're saying, but what if I just don't care? What if I justdon't agree? Now I could tell them that they're being kind of intellectually lazy or I could say that they're not taking it seriously or whatever. But I think these people are being honest and serious. It's like, this is a serious position that needs to be taken into account. You tell them you try to argue in their own system