Abū Zayd's reply uses shawāʾib, a term suggesting impurities, troubles or distressing experiences. It points to the accumulation of hardships rather than necessarily to one identifiable incident.
Grey hair becomes the visible sign of those experiences. The outer change is in his hair and beard; the inner story is his exposure to the uncertainties of worldly life.
This is a poetic explanation within the narrative, not a universal claim that all greying must result from sorrow. The speaker connects his altered appearance to the hardships he has endured.