For me it is the wholesale - "Everyone can do this, it is a moral failing on your part that you aren't already vegan" that irks me.

I have a combination of diseases that mean I could never go vegan, but every single time I have interacted with a vegan online they take the stance that I am lying/wrong and just justifying my choices. Well - My choice is to not die of malnutrition while being tied to a toilet 24/7.

Like yes, random internet vegan - You certainly know my dietary needs better than my doctor and I do...