Why to be offended by somebody's naivety? Why take it personally unless you can't lessen the connection with that person? In my opinion you have lots of choices who to associate youself with and cases when you absolutely can't leave are extremely rare. If you choose to stay you are also partially responsible for keeping yourself in the zone that inspires that kind of meanness in you.

I also believe that there are people whose motives are different than what you may have considered. There are also people whose motives are different than what they may have considered.

I also thought about 1 more reason. There are people who don't like others to be happy. Why? Because they are unhappy themselves and it feels easier when others are unhappy around them as well.