It's just that people aren't replaceable. When they are gone, there is nothing that can fill the space that their particular energy or personality's absence has left empty. That's what makes relationships so special. We are all unique. We matter. No one else can fill that space. And if it's your child?
My son discovered the luminous blue bicycle on his fifth Christmas, next to the tree. That cold morning, I led him along living room and dining room circuits as he got the hang of riding on two tires, with a lot of help from training wheels. He was still years away from any possible attempt at the Tour de France...and his only performance-enhancing drug was the exuberance of a little boy.