I am thin and eat a lot and I don't yather why weight?

Needs testing. Not knowing your age , genetic/family and social history, age, makes it complicated to narrow it down. It can be a problem with absorption of food, intestinal problems, chronic diarrhea , parasites, or a endocrine problem like high thyroid or low cortisol. Smoking or anxiety can also cause weight loss. Need to see PCP to be evaluated and have further testing.