Why does the nipple sink in?

Patient's question:

From a young age

Doctor's answer:

Medical Analysis: Hello: Your symptom is nipple inversion.
Advice: When you were young, wearing tight clothes or other reasons may have caused your nipple to be retracted into the breast. If you are still young, you can gently pull it out during each bath. If you are married, we will provide you with other advice.
Doctor's Inquiry: How old are you? Are you married?