Most of us can only dream of being able to eat whatever we want and not gain an ounce. Being underweight is actually far worse for your health than being overweight.
Being obese is associated with a 50% greater risk of early death. Being underweight is associated with 140% greater risk for men and 100% greater risk for women.
Inability to gain weight can be a symptom of a serious underlying medical condition including eating disorders, thyroid issues, diabetes, cancer, and chronic infections.
Talk to your doctor first, and then look at these tips for gaining healthy weight. Interestingly, many of these tips to gain weight are also ones that will help other people lose weight.