Because in today’s era, I don’t see anybody hunting to survive like the lions in Africa. Meat is simply a non-essential addiction. There is no meat-product for which a healthier vegetarian alternative does not exist.
Meat is non-renewable
Because meat is non-renewable. An animal once killed cannot grow more animals. But a plant, even when uprooted, can grow a new plant from its roots, shoots and seeds. Thus the hunger and thirst caused by meat-eating is much more longer term and intense than that discussed earlier.
Most polluting industry
Because meat and livestock is one of the most polluting industries of the world. Whatever ecological damage we face today can be attributed significantly to this non-essential industry. (refer United Nations document on meat industry being culprit for climate change, land pollution, water depletion and loss of biodiversity at http://www.fao.org/docrep/010/a0701e/a0701e00.htm)
Cause of hunger and poverty
Because meat is cause of hunger and poverty in the world. Just as I myself hate being hungry and under-nutritioned, I feel the same for millions of my brothers and sisters born from same mother earth. If I could have killed myself to satisfy their hunger, I would have gladly done so.
But for all I know, if people stop eating meat and adopt vegetarian ways, they could feed at least 10 times more people using the same efforts and energy. This is based on the principle of energy-pyramid which states that the at least 10 units of vegetation is consumed to prepare 1 unit of meat by feeding the meat-producing animals. Refer any text on food chain or energy pyramid. In fact for most of the commercially produced meat that humans eat currently, energy losses are much higher.
Thus each person who stops eating meat is feeding at least 9 more people apart from himself. What could be a greater form of charity than simply avoiding meat! And what could be a greater sin for me than forcing at least 9 people to die from hunger because I love the taste of mutton! Moreover, meat industry also significantly depletes water.
Since I consider entire humanity as my own family, I cannot sleep peacefully with the guilt of being a cause of hunger and thirst for my dearest brothers and sisters and innocent children in my global home.
Life is Beautiful
Because I consider my life to be dearest to me. I consider life of my near and dear ones to be most precious. I see all humans considering lives of their near and dear ones equally precious. If I kill any of them, I am deemed a murderer because I snatched the most precious gift of life.
So how can I commit the same crime on other species who too have a face and brain like me, who also love their life so much, who also face the same fear when they approach death, who also express happiness and grief like me and my dear ones? Simply because I do not understand their language or consider them less intelligent? By this logic, even killing of mental-patients should also be legitimized. Killing of coma patients should also be legitimized. Eating orphans should also be legitimized.
And since they are not, even meat eating is a crime of same order for me.