What Causes Constipation? (by webmd.com)

Constipation is usually caused by a disorder of bowel function rather than a structural problem. Common causes of constipation include:

Inadequate water intake.

Inadequate fiber in the diet.

A disruption of regular diet or routine; traveling.

Inadequate activity or exercise or immobility.

Eating large amounts of dairy products.

Stress.

Resisting the urge to have a bowel movement, which is sometimes the result of pain from hemorrhoids.

Overuse of laxatives (stool softeners) which, over time, weaken the bowel muscles.
Hypothyroidism.

Neurological conditions such as Parkinson's disease or multiple sclerosis.
Antacid medicines containing calcium or aluminum.

Medicines (especially strong pain medicines, such as narcotics, antidepressants, or iron pills).

Depression.

Eating disorders.

Irritable bowel syndrome.

Pregnancy.

Colon cancer.

In some cases, lack of good nerve and muscle function in the bowel may also be a cause of constipation.

Information Source: http://www.webmd.com/digestive-disorders/digestive-diseases-constipation