Why are some of my friends underweight yet perfectly healthy? shouldn't they be more tired and depressed from the lack of nutrition?

Life: Being thin does not in any way imply malnutrition.

No: Underweight does not necessarily mean undernourished. If you are eating a diet high in fruits and vegetables, you will get many nutrients with fewer calories compared to a diet high in processed foods. In Processed foods, you need more of them to meet your nutritional needs and therefore you eat more calories.