What are the main causes of hemorrhoids?

Hemorrhoids occur when the veins around the anus are under too much pressure, which can be caused by a number of factors, including:   Straining during bowel movements,   Sitting on the toilet for long periods of time,     Chronic constipation or diarrhea,   A low-fiber diet,   Weakening of the supporting tissues in your anus and rectum,   Pregnancy,   Frequently lifting heavy items,   Being obese,   Anal intercourse and Certain diseases such as cirrhosis.