Why does my middle finger hurt

Unknown: Maybe you use it too much. Have it examined by a local physician, possibly sprained or sports related.

Sore finger: You will need an examination and possible imaging to answer that question. Arthritis, sprain/strain, connective tissue damage, anatomic anomaly, trauma, etc? See your General Physician. Referral to specialist Rheumatologist or Orthopedic Surgeon May be indicated.