"...Say Hello to my lil' friend!!!..." - Tony Montana in "Scarface"

I have 2 nephews, both about 4 years old. They don't see each other often - two different cities. I think in their whole long lives of 4 years, they've seen each other a total of 5 times for a few hours each, and we're talking many months between visits. The point I'm trying to make is, they're not best friends or anything, if there is such a thing at age 4.

They both visited a few weekends ago. That's them in the picture. It took all of about 90 seconds for them to start playing together like they were brothers who had grown up together! I'm talking loud shouting, spinning around holding hands, imaginary gunfights, coloring, sharing toys -type playing. It's like, I go to the bathroom, come out and find that they're best friends already. Wow.

I know adults who would haze you for two years before they would decide to let you be their friend. Why can't us grown folk learn how to build relationships like 4-year olds?