"If you live to be a 100 I want to live to be 100 minus one day so i never have to live without you."
-Winnie the Pooh
If you love someone then you don't ever want to live without them. You want to be with them forever and ever. So if they were to live until they were 100 years old then you would want to live to be 99 years old so you would never have to live without them.