What is it so sad that we grow up?

Parents and all always sad when their kids grow up. One mom said she wasn't ready for her son to be 10. I used to babysit two of my cousins in the past when they were 7 and 3 and now they are 17 and 13 which is very ever sad because time goes by and I don't know why I'm so sad about it! STOP GROWING UP! JUST STOP IT!!!! 😢