I've been veggie for 20 years and for the last 6 months have been vegan. Why? My belief that it is a healthier diet and, more importantly, I am opposed to factory farming methods.
All the information is out there for anyone who is interested. A couple of things to check out if you are interested - the documentary "Forks over Knives" (which is on Netflix, but also fairly easy to track down online) and, for those that prefer a book, I'd say one of the best is "How Not To Die" by Dr Michael Greger
I'd add that just as there are healthy meat eaters (ie those that limit their intake and also eat a lot of fruit and vegetables) there are also a lot of unhealthy vegans.
The trick to a healthy lifestyle is quite simple: lots of fresh fruit and vegetables (esp cruciferous), lots of whole grains, lots of beans and legumes, nuts and seeds, and root vegetables - all of which are delicious with the right spices, herbs etc. The stuff that is unhealthy and should be avoided, or at the very least eaten in moderation, includes processed foods, oils, sugar and cholesterol. And regular exercise of course.
I'm not so hardcore that I avoid leather shoes or only drink vegan wine and beer - but perhaps one day though I doubt it.
Forgotten the word cruciferous despite growing most of them over the last 15 years on the allotment. Had to ask the wife who shouted at me for not knowing. They contain all the brassicas and a few more. Called cruciferous because the flowers have 4 petals in the shape of a cross. Thanks and thanks to NSC!
I do think vegetable oils are a necessary addition to a diet and are beneficial to health. Good and bad cholesterol is the new ratio looked at in medical tests.