Why? You ask. Well, here are a few possible reasons for the change:
-- It may be more sanitary. Not exposing your whole hand to your friend's germs is a good thing. Why let your open palm sap up ALL your friend's germs, when your fist might have contact with only a limited number of them?
-- Fingernail grooming may be greatly reduced. Yippee! Groom one thumbnail, and you'll probably be good to go.
-- If a "friend" suddenly morphs into a foe, you will be immediately prepared to smack him or her right in the kisser. Pow!
Conclusion: As one pint-sized 5-year-old kid admonished his dad (who was attempting to be cool by high-fiving his son): "Get with it, dude. A high-five is so yesterday!"