Anyone know what is a reason for lactose intolerance?

Lactose: The cause is the inability or decreased ability of the small bowel to digest the disaccharide lactose due to lack or decrease lactase activity. Lactase is the enzyme that breaks down the lactose to two monosaccharides that can be absorbed. If lactose is not digested in the small bowel it gets to the colon and causes the symptoms. More inf @ http://www.Mayoclinic.Com/health/lactose-intolerance/ds0.