Why sometimes guy can be so insensitive?

Im so thin and im well aware of it. This one guy also always say im thin and frequently ask me to eat a lot. Then one day, we were playing going inside a small and close area and he's asking me to try and go inside and i tell him “if A (because A is thin too) can, ofcourse i can too”

To his response “You are fat. So just try” he say that infront all of my friends, and even got guys there. That guy had more common sense and tell him “you wanna die saying that?”

That hurts me actually, i know he didn't truly mean im fat, but why would you joke about that

Why sometimes guy can be so insensitive?
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