The reasons vary among individuals, a visit with a urologist to discuss your unique situation would likely be useful.

Assuming you mean conditions that INHIBIT sexual performance, they include dieases like DIABETES, LIVER Dz, KIDNEY dz, DEPRESSION and MOOD disorders, CHRONIC INFECTIOUS/INFLAMMATORY disorders, DRUGS( like pain pills, ie-opiates, anti-depressants, psychoactive drugs, and blood pressure meds, NEUROLOGICAL diseases like neuropathies, spinal cord, and brain disorders, ENDOCRINE disorders(thyroid, adrenal, and pituitary), and UROLOGICAL disorders like prostate cancer/surgery and Peyronies dz

Many factors involved...from a physical standpoint adequate blood flow to penis very important , also testosterone level. Relationship status also critical

Excellent health, including mental health
DR Jagdeo