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the traditions, savoir-faire and craftsmanship associated with meat, dairy, cheese and fish pointless, irrelevant and outdated.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, for many people, being vegan seems to make it difficult to share a meal with you, and everyone knows that social relationships are strongly tied 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 2014, I was the only vegan living in a five-person house share. This sometimes isolated me because I couldn't share a barbecue or a plate of spaghetti bolognese with my roommates. However, the benefits of veganism for the environment, animals, and myself outweighed this minor social inconvenience.
Open-minded people will always accept your diet, and there are many recipes that vegans and non-vegans can share.
If you are wondering why people become vegan, you can find more information here: Debate a vegan.