"Increased straining during bowel movements, by constipation or
diarrhea, may lead to hemorrhoids.[6] It is thus a common condition
due to constipation caused by water retention in women experiencing
premenstrual syndrome or menstruation.
Hypertension, particularly portal hypertension, can also cause
hemorrhoids because of the connections between the portal vein and the
vena cava which occur in the rectal wall -- known as portocaval
anastomoses.[7]
*****Obesity***** can be a factor by increasing rectal vein pressure.
Sitting for prolonged periods of time can cause hemorrhoids. Poor
muscle tone or poor posture can result in too much pressure on the
rectal veins.
Pregnancy causes hypertension and increases strain during bowel
movements, so hemorrhoids are often associated with pregnancy.
Excessive consumption of alcohol or caffeine can cause hemorrhoids.[8]
Both can cause diarrhea. Note that caffeine ingestion increases blood
pressure transiently, but is not thought to cause chronic
hypertension. Alcohol can also cause alcoholic liver disease leading
to portal hypertension."
http://en.wikipedia.org/wiki/Hemroids
