Epididymitis: This is the most common cause of testicular pain in young men, often resulting from a bacterial infection, typically Chlamydia or Gonorrhea. The pain is usually accompanied by swelling and redness of the scrotum.

Orchitis: Inflammation of one or both testicles, usually caused by bacterial or viral infections. It can present with testicular pain, swelling, and fever.
Testicular torsion: Although more of an emergency, it's a possible cause of acute testicular pain. It occurs when the spermatic cord becomes twisted, cutting off the blood supply to the testicle.
Varicocele: An enlargement of the veins within the scrotum, similar to varicose veins. It can cause a dull ache or discomfort and is more common on the left side.

Testicular Torsion: As mentioned, this is a surgical emergency. If not treated promptly, it can lead to the loss of the testicle. The pain is usually severe and of sudden onset.
Testicular Cancer: Although less common, it's crucial not to miss. Pain can be a symptom, though it's more often discovered as a painless testicular mass.
Appendicitis (with referred pain): In some cases, appendicitis can refer pain to the testicles, especially if the appendix is located near the right testicle.

Henoch-Schönlein Purpura (HSP): A form of vasculitis that can cause testicular pain among other symptoms like purpura, joint pain, and gastrointestinal symptoms.
Spermatocele: A cystic structure that contains sperm, usually causing a painless mass but can occasionally be painful.
Scrotal cellulitis: An infection of the skin and tissues surrounding the testicles, which can cause pain, redness, and swelling.