(Brooklyn NY )
Vegetarianism (and later, veganism) for me started about 10 years ago when I was in undergrad. I had been an anti-death penalty advocate since I was 16, motivated by the conviction that ending suffering of living beings is an ethical credo I wanted to ascribe to. However, I hadn't made the connection between humans and animals until I was 19 or 20, when I had a vegan bring to my attention the suffering of animals by giving me literature about the factory farming industry.
I tried to imagine EVER hurting an animal like that, and couldn't! So why should I allow that to happen? The incredible cruelty was enough to make me remove meat from my diet. Amazingly, my future spouse was inspired by my non-preachy example (and also has been vegetarian for 8 years. So you can have a ripple effect with your humanity!!
Recently I made the switch to veganism because the dairy and egg industries are equally as appalling as the meat industry, but I believe firmly that each person should eliminate suffering in their diet as much as possible. So, if you can't give up cheese, at least try to reduce intake of other animal products which cause suffering.
To those who would say you can't be healthy on a vegetarian diet or vegan diet---I have run 12 marathons and ultra marathons and have done numerous triathlons. It is possible, and gives a great example to live a plant-based, cruelty free life!