Why can’t I gain weight?

Okay so basically growing up I’ve always had a really huge appetite and I was always a heavy eater but I’ve always been thin and really skinny. I literally eat the same amount of food as a really obese person would eat. Growing up I had always been skinny with a completely flat stomach and a lot of people say that I’m too skinny. And it’s like no matter how much I eat I can’t gain weight and I’m still super skinny. I just wanna know if there’s anything wrong with my body or if my metabolism is too high.
Why can’t I gain weight?
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