He may have caught a disease from the female he was involved with, a virus perhaps, even a parasite. It is my opinion that viruses such as those that infect the nerves can affect behavior, similar to how the rabies virus affects the behavior of an animal. Herpes, aids, maybe even bacterial infections as well or parasites.
In the natural health world, diseases and disorders are often blamed on infections, viral or bacterial, or parasites. You won't hear much of that from your doctor or from standard medical practitioners.