Skinny people who eat junk food?

I'm not asking how they stay thin cuz I know that they just exercise more than the average fat person who eats junk that doesn't exercise. But I just want to know why people who brag about being skinny like "Oh I'm skinny. I wont gain a pound cuz I have high metabolism so it's alright if I eat three slices of pizza." I'm just wondering. What about your health? Being healthy is more important. I see kids eating a bunch of junk food like soda, chips, and candy that claim that they wont gain any weight from it. But do they bother to think about their health? Eating junk food doesn't make you feel good. It just allows you to be be energized for a while and then that energy goes away and you feel crabby.