We attach labels to people and situations because it is easier to identify them, and it provides a feeling of control. The most hurtful are those which initially are not very mean but which are repeated all the time.
Phrases that concern the field of mental health, like “you are bipolar”, or those that could be described as sexist like ‘are you on your period?’ can be particularly hurtful. The closer we are to the other, the more we risk hurting them.