I actually was vegetarian before I became a Buddhist. I was interested in Zen Buddhism then, but hadn't started practicing Zen until a couple years after I stopped eating meat.
In 1999, the Makah tribe in Washington State, USA, got legal approval to hunt whales in the tradition of their ancestors. After they successfully hunted and killed a gray whale, I was pretty conflicted about the issue. On one hand, I support of the rights of Native Americans under the treaties that the US government has agreed to. On the other hand, I love sea mammals. I used to have incredibly peaceful and joyful dreams of swimming with whales and dolphins. I had an intuitive sense that the Makahs were on the wrong side of this issue, morally if not legally. But I couldn't justify it in my own mind.
Is all hunting wrong? I don't think so. I imagine that if I were a Makah living without agriculture in the fifteenth century I would join the hunt proudly, and join with my tribe in gratitude to the whale for giving up its life to keep the tribe alive.
But I couldn't shake the idea that this killing was wrong, because none of the Makah in today's society would suffer, even a little, if they didn't kill this whale. And that led me to look at myself. What's the difference between a cow and a whale? I imagine the whale is probably quite a bit smarter, but how exactly do you measure that? And is our intelligence the only thing that makes us valuable as living beings? How do I justify the killing that's done on my behalf?
So I became a vegetarian, as an experiment to see if I could do it and stay healthy. Agriculture provides us with plenty of nutritious food. I have been vegetarian for almost seven years now, without any dietary modifications besides giving up meat. I get plenty of protein from beans, grains, and dairy products, and I'm in great health. And I'm happier with my eating habits.