“Why do I only have 1 testicle?”

Undescended or Congenitally absent testis, in the later case the hemiscrotum would be hypoplastic

If you only have 1 testicle you need to be examined by a urologist for a possible undescended testicle.

Born with one testicle is uncommon, but clinically, severe testicular atrophy (shrinkage) after mumps in the young age could be possible. Yet, to disclose if your concern is significant to your life/health, it is a good idea to see an able, trustworthy urologist. Before visiting the doctor, it is a good idea to prepare yourself for a medical consultation; to do so, please go to "Sick" What To Do?" at http://www.formefirst.com/eNewsletter06.html. Besides, inside www.formefirst.com, there are a lot of articles on life, reality, and urology. If what you read about may be too heavy for you, it is a good idea to seek help from your parents or teachers or trustworthy friends.
Here come my best wishes for your health and happiness in 2020 and well beyond.

The testicle probably torqued during embryonic development, or the testicle did not form at birth, which is a congenital problem. Typically in this situation, the other testicle hypertrophies (gets larger) to take over the function of the missing testicle.

Dr. Niko Lailas