What causes erectile dysfunction?

Currently, virtually any man who wishes to have an erection can obtain it, regardless of the underlying cause of his problem. Many reasonable treatment options exist. Your first step is to find a well-trained, experienced, and compassionate doctor who is willing to take the time to understand you and fully examine you to discover the cause and discuss the treatments available to you.
 
Your doctor will want to rule out any other causes for your concern such as high blood pressure, prostate cancer, vascular disease, and diabetes. 
 
By seeing your doctor, you may very well be saving your life if the doctor detects - and treats - a life-threatening illness. Often, you can restore your sexual health by treating a condition such as high blood pressure with diet and exercise or controlling your diabetes. 
 
For some men, erectile dysfunction develops with age or may be related to depression or another psychological cause. In these cases, psychological counseling with you and your sexual partner may be successful.  
 
Medications can cause ED, especially drugs you might take to control blood pressure or depression (antidepressants) (see Impotence/Erectile Dysfunction for a list of medications that may cause ED). Anti-ulcer drugs can be a cause, as can alcohol or drug abuse. ED is a side effect. Talk with your doctor about medications that might not cause this side effect. Do not just stop taking your prescribed medication.

Other causes include damage to the erection bodies in the penis; diabetes; various hormonal disorders; blood flow problems; psychological factors, such as depression; and surgical complications from abdominal, pelvic, or back surgery.