Why do some people overreact to someone being vegan?
Food has a lot to do with culture, habits and traditions. Going vegan makes all the traditions, savoir-faire and craftsmanship associated with meat, dairy, cheese and fish pointless, irrelevant and outdated. Some non-vegans may find this offensive, even insulting. Because it questions their habits, their culture and their traditions around food. It questions their identity, it questions who they are.
Furthermore, being vegan makes it difficult to share a meal with you, and everyone knows that social relationships are strongly linked to the acquisition, preparation and sharing of food. Some people may think you're putting some distance between you and them, and they may end up rejecting you.
In 2016, I was the only vegan living in a flat share with five people and it sometimes isolated me in the house because I couldn't share a barbecue or even a plate of spaghetti bolognese with my flatmates. The benefits of veganism to the environment, animals and myself outweighed the minor social inconvenience, so I was happy with my choice.
Nice people will always accept your diet and there are many shared recipes that vegans and non-vegans can share.