Tree houses are awesome. I'm not sure what makes them so cool - perhaps it's the "being at one" with nature aspect, or maybe it's the "use your imagination" aspect... Maybe it just has to do with "living" in a tree, as if one is stranded on a desert island and has needed to build a shelter to survive (I think this might relate to the "imagination" aspect of it all).
I used to spend a lot of time in the tree house that my dad built in the backyard of my childhood home... And I'm fairly sure that I could live in one permanently, if it were to look something like this: