First, I have high expectations for you. People often say that "hate iron and not produce steel", so the meanness shown is beyond the understanding of ordinary people, and certainly not to outsiders.
The second reason is that you can wait for something. If you have a good relationship, you will be negligent in etiquette. There is an old saying in China called "cooked". It means that people who know each other well don't have to stick to etiquette.
The third reason is that the cost of hurting close people is low. This is easy to understand. After the words of colleagues or friends are hurt, they will form a gap, which is difficult to make up. Relatives are not. A few words can make up for it.