sharing

Why is it so hard to teach a little kid (or even adult) to share? Right now I'm trying to teach my little 19month old Kali to share and she wants nothing to do with it. Both her older brothers are pretty good with sharing after a long process of several years. Maybe because she is the youngest, she gets things taken away from her so often, that she will do anything not to have to share. Lately, in the front driveway, I've been having all 3 of my kids take turns in a little red car while I push them down the sidewalk. But when it's Kali's turn to get out....ohh my! The nieghbors probably think the tornado warnings have gone off.
But I have found that life is better when you share. When you accomplish something great, you want to share that with someone. Or how about enjoying an cool experience together. Maybe it's an ice cream, a cold glass of lemonade or a ride with the convertible top down. There have been times in my life where I have hoarded something all to myself.....and honestly it didn't taste as good nor seem as fun.
What about you, are there certain things you won't share? If so, why not?