Non-violence leads to the highest ethics, which is the goal of all evolution. Until we stop harming all other living beings, we are still savages. -Thomas Edison
Environment - Growing animal protein is land, water and resource intensive. For example, much of the destruction of Amazon rainforest is as a result of beef production for the American hamburger market. Producing animal protiens is inefficient, more protein is consumed by the cow than you can harvest from its carcass.
Every creature is better alive than dead, men and moose and pine trees, and he who understands it aright will rather preserve its life than destroy it. -Henry David Thoreau
Health - There are many diseases that are uncommon in long term vegetarians: heart disease, bowel cancer and multiple sclerosis are examples. Further, the higher you eat on the food chain the more concentrated toxins become - pesticides, herbicides etc are much more concentrated in flesh than they are in the surrounding environment. Most animals raised for human consumption are also heavily treated with hormones, steroids and antibiotics.
Nothing will benefit human health and increase chances for survival of life on Earth as much as the evolution to a vegetarian diet. -Albert Einstein
Ethical - Animals raised for consumption are rarely kept or treated with respect. The very clever The Meatrix flash movie probably covers this topic better than I could. I don't believe that animals should be accorded the same rights or treatment as humans but I do believe they are entitled to a life of dignity and should be honoured for their sacrifice.
A man is truly ethical only when he obeys the compulsion to help all life which he is able to assist, and shrinks from injuring anything that lives. -Albert Schweitzer
These are complex topics and my bullet points hardly do them justice, I am just giving a flavour of my reasoning. If you want to read more a place to start is Diet for a New America by John Robbins, also worth reading is Diet for a Small Planet by Frances Moore Lappe.