Pencil: I'm sorry
Eraser: For what? You didn't do anything wrong.
Pencil: I'm sorry because you get hurt because of me. Whenever I made a mistake, you're always there to erase it. But as you make my mistakes vanish, you lose a part of yourself. You get smaller and smaller each time.
Eraser: That's true. But I don't really mind. You see, I was made to do this. I was made to help you whenever you do something wrong. Even though one day, I know I'll be gone and you'll replace me with a new one, I'm actually happy with my job. So please, stop worrying. I hate seeing you sad.
Parents are there always for their children, cleaning up their mistakes. Sometimes along the way, they get hurt, and become smaller / older, and eventually pass on. Though their children will eventually find someone new (spouse), but parents are still happy with what they do for their children, and will always hate seeing their precious ones worrying, or sad.
Pencil [as children], it pains them to see the eraser [their parents] are getting smaller and smaller each day. For I know that one day, all that I'm left with would be eraser shavings and memories of what I used to have.