Hemorrhoids, as such, are not an ailment. They are merely blood vessels within the anal canal. Generally, they serve to help with the passing of solid waste. It is when hemorrhoids get sqeezed and become bulbous, thereby actually obstructing solid waste that they become a problem.
In a nutshell, hemorrhoids (actually the ailment manifested from hemorrhoids) are polyps or bulbs of blood within the veinous tissues. Sometimes the blood dries and becomes clotted which can compound the problem even further.
So to further the discussion as to what causes hemorrhoids, anything that can cause the veins to bunch up into bulbous tissue can cause the development of hemorrhoids. The question then becomes: what can cause the veinous tissue to bunch up like that?
Excessive Weight - an overweight body will put pressure on the lower pelvis, and therefore the rectum. This may be due to over-eating, under exercising or something else such as pregnancy. In fact pregnant women often develop hemorrhoids.
Bad Eating Habits - a bad diet can cause faeces to build up within the colon. This can cause many different problems including the obstruction of the passing of solid waste (which causes even more faeces to build up within the rectal canal). A lack of certain foods such as fiber can cause at least as much trouble as eating bad foods.
Stress - people who are subject to high levels of stress tend to suffer more from hemorrhoids.
Rushing The Elimination Process - not being patient and 'pushing' during bowel movements can cause hemorrhoids.
Not Eliminating Waste When Needed - we all find times when our bodies tell us we need to go to the bathroom when it is inconvienient. The more often we wait or 'hold it' instead of going when our bodies signal us, the higher the risk of developing hemorrhoids.
Diarrhea or Constipation - any problems with bowel movements can cause undue pressure on the bowels and thereby cause hemorrhoids.
Medical Conditions - certain medical conditions such as heart or liver disease have been known to contribute to the development of hemorrhoids