Are some people just more prone to diarhia?

YES...: Those with 'irritable bowel syndrome', have more diarrhea, there is treatment for this. Diarrhea can be a symptom of systemic illnesses, malabsorption syndrome, anxiety, food allergy, etc. Food, drink, beverages which cause high osmolar environment in the intestine, draws more fluid into the lumen causing diarrhea. Check with your doctor if it is bothering you.