I think it must be a primal childhood urge. I refer here to the need to literally rise above it all and to elevate oneself to a place of spatial superiority. In the feline world, it serves to buttress the cat's feeling of dominance over dogs, nay, over all lesser beings- that ability to effortlessly ascend to the upper reaches of whatever tree happens to be handy, and to perch regally and survey the scurrying of the inferior world below. I see this happen to my cat Moosh- normally a skittish, nervous type, on acquiring her favorite tree perch in my magnificent scarlet willow, she transforms into a tiny lioness, fearing naught, oblivious to earth-bound perils, and superior in every way.
As kids, I think we naturally gravitate (or would that be "de-gravitate"?) to this tendency. We start out as drooling floor-bound crawlers, cut our teeth as we toddle about with our tummies off the ground, and eventually get the urge to mount anything climb-able. Since age one, I have had a scorpion-shaped scar below my lower lip, as evidence of this infant urge. Left unattended for a brief moment, I managed to stand up in my stroller, which tipped over backwards as I attempted to climb the push-bar, launching me face-first into a nearby coal bucket, thus marking me for life.
So I think it is only natural for children of all ages to have a fascination for tree-houses. I distinctly recall two such structures from my own formative years. One was inherited with the house my folks bought when we moved to Rock Hill, South Carolina, at which point I had already reached the ripe old age of sixteen, and was thus far too advanced to indulge in such puerile pursuits, and so left the tree house to my younger siblings. You will have to ask them about that particular structure, but I recollect that it was quite elaborate compared to our previous one, having a roof and side rails to keep the climber safe. The one I remember best was one our dad built in an adjacent lot when we lived in Seneca, SC. It was a mere elevated platform, constructed about six feet off the ground, with no kind of restraint or safety devices. My dad, while a thorough and loving provider, was not a coddler. I don't recall any broken bones resulting from rapid and unrestrained descent, but correct me if I am wrong, my sibs.
It was not until after I had developed an interest in architecture and gone on to study it and become degreed in that field, that I had occasion to see the fruits of childhood tree-dwelling grown up. Those serious designers who sally forth on such projects generally do so with fun mainly in mind, and I love coming across the results. To wit: this posting about tree houses from Dezeen. If you follow the links therein, each tree house is thoroughly (enough) documented and described. And here is another link to some weird and wonderful tree-top abodes.
Maybe I'll just go out and construct a nice tree house in that scarlet willow.... I wonder if Moosh will forgive me.