I have literally stopped talking to a sibling becauase they think it is funny to make jokes about my veganism, or ask if I am still vegan (for life), or if I miss the taste of meat (no, I find the smell nauseating).
It is a lack of respect for life choices, made more loathesome because my choice is made on an ethical foundation, not a whim.
Honestly, that is just one reason. They are a jerk and a bully, like everyone else who comes on here feeling the need to dismiss veganism or claim feeling attacked by it.
Just like other kinds of prejudice, these negative reactions betray a profound level of ignorance that go beyond infuriating to pitiful.