Being vegan is not a belief system, its simply a more narrow form of vegetarianism. Vegan means you dont eat any animal products, this includes butter, milk, eggs and cheese. Vegetarians dont eat meat, but they may eat eggs, cheese etc lacto/ovo vegetarians. There are pesce vegetarians that eat fish. I adopted a vegan lifestyle after a heart attack and quadruple bypass at 41 because I had a genetic high cholesterol issue and wanted to everything I could to bring it down. I remained vegan for about 2 years but found that it was very difficult to maintain, forget about eating out. I once explained to a waitress that I was vegan and she told me that "there is nothing on the menu that you could eat" I eventually started eating fish and after several years now will eat turkey and chicken. Still no red meat after 13 years.
I dont ever remember being preachy about being vegan and did not talk about it unless asked. I think your experiences with vegans are due to the fact that vegans are usually much more dedicated to their beliefs as maintaining that lifestyle requires a certain level of commitment.
I think there are probably a lot of people these days that proclaim veganism because its trendy. Probably less people that are vegan because of animal cruelty concerns but most vegans probably because of health issues like myself.