Epididymitis: This is the most common cause of testicular pain in young men, often resulting from a bacterial infection, usually sexually transmitted. The symptoms can include pain, swelling, and redness of the epididymis, which is the tube at the back of the testicle that stores and carries sperm.

Orchitis: Inflammation of one or both testicles, usually caused by bacterial or viral infections. It can cause pain, swelling, and tenderness in the testicles.
Testicular torsion (partial): Although testicular torsion is more of a "do not miss" diagnosis due to its urgency, a partial torsion could present with less severe symptoms and might not always have the classic signs of severe pain, nausea, and vomiting.
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 torsion: A medical emergency where the spermatic cord becomes twisted, cutting off the blood supply to the testicle. It requires immediate surgical intervention to save the testicle.
Testicular cancer: Although less common, testicular cancer can present with a painless testicular mass, but sometimes it can cause pain, especially if there's bleeding or if the tumor size increases rapidly.
Appendicitis (with referred pain): In rare 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 affects blood vessels and can cause testicular pain among other symptoms like rash, joint pain, and abdominal pain.
Spermatocele: A cystic structure filled with sperm that arises from the head of the epididymis. It usually doesn't cause pain but can if it becomes large or inflamed.
Idiopathic scrotal edema: A condition characterized by the sudden onset of scrotal swelling and redness without an identifiable cause, often seen in children but can occur in young adults.