1. I can't understand why...:
a sentence pattern used when angry or surprised at something or someone
- I can't understand why you like this TV show. It is so boring.
- I can't understand why we have to wear suits to work. We don't meet customers, so it is not necessary.
2. to be rude:
to behave in an inappropriate way; to not be polite
- He is rude.
- It is rude to stand very close to people in my country.
3. to be mean:
to be not nice
- She always yells at everyone. She is mean.
- He called me fat. He is so mean.