Why do I feel so guilty...

Why do I feel guilty for being insecure about being skinny. Like the bottom half of my body is fine, my legs etc but I'm so insecure about how skinny my upper body is. Like I exercise and eat a lot but I struggle to put on weight on my upper body like my arms are so lanky in photos and it makes me look so WEIRD. And we went to the beach once and my friend took a photo of me and u could see like the bone in the middle of my chest, like between my boobs lol. And when I raise my arms u can see my ribs (but I think that's more the fact my rib cage sticks out a bit) And the fact I'm only like a size A doesn't help either. But then I feel guilty and stupid for being insecure about it when my friends are always saying that they wish they had my metabolism idk bleugh