I have more than once heard/read people disagreeing with veganism because they like to see farm animals in the fields! Also so concerned about what would happen to existing animals like dairy cows or pregnant sheep if everyone went vegan tomorrow and how cruel that would be. Overbreeding overbred animals to live short, uncomfortable lives and then exploiting and killing them is therefore for the welfare of the animals themselves and the prettiness of the countryside apparently.
I wouldn't mind discussing it but the overwhelming amount of discussions are not in good faith, it's non-vegans waiting for a 'gotcha moment' like 'frankenfoods', air miles on avocados, wildlife killed when harvesting crops, sudden intense concern for your health and macronutrients (even if you both used to do coke in portaloos at festivals), pets, or keen interest in your shopping habits to see if they can blame you for child slavery or blood diamonds or something.
It is tedious. A pp has it right that it is a pre-emptive strike in case you try to guilt them.
I did find the PP getting upset that her family will never be able to enjoy a goose fat roast potato together rather dramatic. It's spuds, you will be fine. And if you don't enjoy or are not capable of catering for different diets, then just don't - go out, order in, make them bring something. Implying people should give up their tastes and beliefs because you want to watch them all eat a goose is rather entitled.
I've never encountered one, but I am sure preachy vegans exist; their approach may not work on everyone but I'll never not admire people who actively stand up for what they believe is right - environmentalists, civil rights, Palestine, veganism etc. If you don't agree and are secure in your viewpoint, carry on and get on with your life (or counter protest) but if their activism bugs you, look at why. It's healthy to change your mind when given more information.