Please Step Back

I love the self checkout at the grocery store. I just love scanning. It was my favorite part of working retail. What I don't enjoy at the self checkout is the person behind me who feels compelled to stand right next to me while I'm trying to scan my purchases. I never have this problem in the regular checkout. Why does this person figure that this is acceptable behavior in the self checkout? It's not going to make things go faster and it's waaaaaaaaaaaaay in my personal space.