This has to be the all out question I receive the most from family, friends, co-workers etc. Everyone always wants to know your reasons, some will try to change your opinion. In the long run your reasons are just that yours it is a personal choice that you make, and shouldn't matter to anyone else.
My reasons tend to change over the years and the new facts that come up all the time. For me it started off simple I was over weight, 100lbs over weight. So at that point I decided I needed to make a change. My family background is Italian and French so meat was a huge factor in my diet growing up. I was the type of child who would eat meat and carbs mainly, I really don't think I ate many vegetables aside from salad.
When I finally got sick of the way I felt and looked the first thing to go was red meat, I started to eat only tuna and chicken. Then the chicken came out of my diet because I started to get sick of the smell. This probably has to due with the fact my grandparents had a small family farm when were younger. They had chickens and a few turkeys, they pretty much lived off of what they grew and raised like they had done in Italy. There were plenty of days when I was at my grandparents house, I would go downstairs and my grandmother would be cleaning a chicken to cook. That smell alone was enough to haunt me for awhile.
Slowly but surely I stopped eating all meat and I began to really feel amazing. I never felt this great in my whole life I was down over 100lbs and my health was back on track. While being vegetarian I started to look into veganism more and more. When I would think about what eggs and milk really were I would get grossed out. It is really weird we are the only species to drink the milk of another species, let alone the fact we are the only species that drinks milk past weaning. So eggs and dairy became something that I no longer wanted to eat. I began to start eating vegan though if people ask I will tend to say I eat vegetarian because there still is some stigma associated with being vegan. I have a rule though you don't judge me and I won't judge you. I am not the type of person who will pretend to be morally superior to another, your life is that yours and you are free to live in the way you see fit.
For my own personal choice it really has encompassed everything. For health reasons, for animal rights reasons, just for the sake of compassion. It starts to become really amusing for me though when Physicians tell me I should eat meat for health reasons. North American culture really is funny, if you are low on Iron the only thing a Dr will advice you is to eat more red meat. Though there are plenty of plant based options for Iron. The other secret reason I will let you in on of why I became a vegetarian/ vegan is that in the event the planet is run by apes, or any other event where animals take over the world come to fruition then I am banking on the animals taking pity on the humans who have not and would not harm them. Obviously the last reason is the best reason.