Underweight but scared to gain any weight, is something wrong with me?

What's your fear?: Healthy people do not set out to gain weight. Men want a good amount of skeletal muscle; most women want to build some as well. Men need no body fat. Women need only a little. Take up some fitness activity that you enjoy and your body weight will set itself where it should be. There's no reason to try to put on fat to satisfy the subscientific bmi charts -- you should avoid this.