Epididymitis: This is the most common cause of testicular pain, especially in young men. It is usually caused by a bacterial infection, such as chlamydia or gonorrhea, and is characterized by pain, swelling, and inflammation of the epididymis.

Orchitis: Inflammation of one or both testicles, usually caused by a viral or bacterial infection. It can cause severe pain, swelling, and redness of the testicle.
Testicular torsion: A twisting of the spermatic cord that cuts off the blood supply to the testicle, causing severe pain and swelling. It is a medical emergency that requires prompt treatment.
Varicocele: An enlargement of the veins within the scrotum, similar to varicose veins. It can cause a dull ache or feeling of heaviness in the testicle.

Testicular rupture: A serious injury that can cause severe pain, swelling, and bruising of the testicle. It requires immediate medical attention to prevent further damage.
Testicular cancer: Although rare, testicular cancer can cause pain, swelling, or a lump in the testicle. It is essential to rule out this diagnosis, especially in men with a family history or other risk factors.
Appendicitis: In some cases, appendicitis can cause referred pain to the testicle, especially if the appendix is located near the testicle.

Spermatocele: A benign cyst that forms on the epididymis, causing pain or discomfort in the testicle.
Henoch-Schönlein purpura: A rare condition that causes inflammation of the blood vessels, leading to pain, swelling, and bruising of the testicle.
Polyarteritis nodosa: A rare autoimmune disorder that can cause inflammation of the blood vessels, leading to pain and swelling of the testicle.