The exact cause of IBS is unknown but it is considered as the disease of the gut or digestive tract.
For easy explanation, it can be said that the affected person's gut or digestive tract becomes more sensitive. Because of this sensitivity, a person experiences pain originating from the digestive tract. Due to increased sensitivity food passes either slowly or too quickly causing constipation or diarrhea respectively. So the causes may be considered as slow or over activity of the digestive tract muscles and nerves, sensitivity or intolerance to certain foods, infection of the digestive tract, decreased pain threshold.
Emotional stress plays a very important role in developing and worsening the IBS symptoms. Stress may be in the form of frustration, anger, disappointments, etc.