The most common cause of erectile dysfunction is vascular disease. ED and cardiovascular disease share many of the same risk factors.

These include:
Smoking
Lack of physical activity or exercise
Obesity
Hypercholesterolemia (high cholesterol)
Metabolic syndrome

A brief summary of other causes of ED is shown below:

Neurological causes (Nerve problems): Stroke, diabetes, multiple sclerosis, Alzheimer’s disease, Parkinson’s disease, injuries and tumours to the brain or spinal cord or any conditions which affect the nervous system. These conditions affect the ability of the nerves to pass on ‘messages’ between the brain and the penis.

Inflammatory causes Inflammation of any of the following: The urethra, the prostate, the bladder and the testes.      Inflammation causes swelling, blockages and pain which will affect erectile function.

Psychological & Psychiatric causes: Performance anxiety, relationship problems, stress, depression and schizophrenia. These conditions cause a loss of libido and affect the chemicals which cause erections.

Mechanical causes: Any deformity of the penis (e.g. Peyronies disease), obesity (being extremely overweight). Damage to the structure affects its functioning.

Surgical Prostate surgery, pelvic surgery (colon and bladder). Can cause damage to the nerves and arteries which supply the penis.

Trauma: Pelvic fractures or spinal cord injury. Causes damage to the nerves and arteries which supply the penis.

Drugs                                                                                                                                                                                                Many prescription drugs can cause ED. These include; blood pressure drugs (anti-hypertensives), drugs for heart failure, antidepressants, sleeping tablets and pills for anxiety, anticholinergics, hormones,  H2 Antagonists and Cytotoxic Agents. Drugs can cause ED by the way they affect the brain and nervous system and though their effect on the blood vessels.

Recreational drugs: Alcohol, tobacco, cocaine, ecstasy,  marijuana. These drugs can affect the nervous system and may interfere with blood flow to the penis.

Physical endurance. Recovery after a heart attack, diseases affecting the lungs, anaemia, systemic diseases and liver and kidney disease and sleep disorders. These conditions affect the body’s ability to cope with the physical exertion of sexual activity.

Endocrine Disorders

Low testosterone
Hyperprolactinemia
Thyroid disorders
Type 2 diabetes

References:
Smart D. The SASHA Guide to Erections and Erection Problems. © Ronin-Do (Pty) Ltd 2004.
Lissoos I. Know your manhood. KYB Pocket Books 1984.
Lue T, Giuliano F, Khoury S et al. Clinical Manual of Sexual Medicine Dysfunctions in Men. 2004 Health Publications LTD