Well I don’t know either but here’s what I think:

I argue that each child, growing up surrounded by nature and trees, will at some point come up with the idea to build a treehouse for and with his friends. The desire for a place to withdraw from the grown-ups world, where you can create and live your own world, that way to learn, grow and become oneself while playing, seems inherent. A crotch in the climbing-tree or a place in the treetop appears very attracting, being an ideal spot for such a place. Tucked away behind the leaves, aloft and not accessible for just everyone, the tree offers shelter and the promise to keep any secret. He becomes a friend you trust implicitly and also an accomplice for any plot; he stands and supports and holds the space which makes him so precious for us.

“If we can’t fly with the birds, at least we can nest with them.“ This phrase from Pete Nelson, him also being a friend, illustrates a human longing, which makes also the grown-ups dream of treehouses – the longing for freedom, for a place far from any duties and necessities. For a place of peace and the feeling of security, where you can turn inward and discover other, new perspectives. Maybe in a treehouse you realize, that through another perspective, the observed itself turns into something much more favorable. Maybe, only through the security of the tree and his offer to share the load, a room previously locked suddenly opens. Maybe you wake up after a night under the canopy, replenished and happy, and climb back down to the ground. Maybe you read a book, look at the nature, listen to the rain, on the roof and through the leaves, maybe with your beloved, maybe a treehouse just produces more romantic thoughts than you ever believed of being carrying in you. In any case you will be shown the difference between boredom and leisure, for some in unexpected clarity, quite plainly. A „have you done your homework already?“ or a „ have you been to the post office already?“ becomes a distant, barely noticeable echo from ancient times and with a little dose of luck you may notice your potential of mellowing with age already, though still being a child.
You realize that being „above“ is worthwhile and important, not as „above others“, but as „above“ in terms of “maintaining an overview”, „being above things“ and being „master of the own situation“. „Above“ as an elemental goal everyone can attain. Only by having an overview it’s possible to spot and identify differences, to learn how to distinguish and that way separate the important from the less important things, to stand above it all. A treehouse provides us with the space we need to do all that. This, besides many others, could be the reason we build treehouses.