Medical reasons why i cannot gain any weight?
A medical reason for not being able to gain weight would be due to hypothyroidism or due to genetics which explain that you have a fast metabolism rate. It could also be because of a low blood sugar which is also known as hypoglycemia. You should consult a doctor since you may have an underlying condition or disorder. For more, check out http://www.livestrong.com/article/438633-why-if-youre-constantly-eating-cant-you-gain-weight/.
High Metabolism? Or anorexia nervosa / bulimia.
Fast metabolism I was like that too until I started working out.