(1) They always talk about themselves:
People who are really nice will not keep on talking about themselves always. Talking about oneself each and every time implies that the person is selfish and is always concerned about their own needs. Such persons who can’t even think of a world beyond themselves cannot be nice.
They might not talk about negative things directly but the very fact that they are not interested in listening to what others have to say shows that they are selfish and egoistic. Since they can’t talk about anything or anyone apart from themselves or things concerning themselves, they cannot do anything which will not add value to their lives. So, they cannot be nice.