Mental Intruder Theory

Following on from m's initial statement that It was born after the viral meme of The Slender Man reached its peak. However, I propose that we didn't come up with the meme. It used us. Elements of The Slender Man have been rampant throughout history, increasignly so in recent times.
 
The Tall Man from Yahtzee's Chzo mythos. A tall, faceless demon that can seemingly teleport by existing on a different plane, existing only to kill, brutally.
Jack Skellington from Nightmare Before Christmas. A tall, suited figure that is apparently the most terrifying creature in all realities.
The popular game The World Ends With You. Here it is more abstract - Reapers follow specific rules to hunt the participants of a game, unable to fight near specific symbols and, after toying with the prey for a week, move in for the kill on the final day.

Something has been trying to utilise The Philip Effect before Slender Man. These details are present in so many different medias. The bottom line is that there is a strong possibility that this creature has invaded the collective subconscious of humanity, and has only just achieved form.

Though it is possible that we somehow created him using all of these snippets of Fear. However, I doubt it - few ever played the Chzo games, and fewer still make the link between the two antagonists.

Regardless, these tie in to other aspects that actually have some bearing on the current situation.

The first is The Rules. So far, confirmed weaknesses have been asserted to be part of It's design. Fooling it, outsmarting it. However, it is more than likely that they're looking at this the wrong way. Apply Occam's Razor - the simplest answer is not that you have discovered ways of out-thinking It. Rather, you have inadvertently discovered the rules by which it is bound to. The height factor, for example, could tie in to a forbiddance from going higher than a certain point. The Operator Symbols function much like the reaper's ensignia from TWEWY - it creates a "safe zone," so to speak. Every means we have of avoiding It is just a means of playing by Its Game.

Why, then, does it follow these rules? Is it a side effect of being born through the flexible contours of human imagination? Possibly.More likely solution.

To sustain its manifestation in the world, It requires people to believe in its existence. And using Fear is the most effective method of sustaining this belief - fear's feral nature is unquestioning. You fear something because it is a threat to you. You don't quesiton it, you simply fear it. And that is why he abides these rules. The perfect recipe for one terrified human. The stronger the fear, the more solid the manifestation. Consider this: Why does it take so long for him to begin Touching people? And why does it take further still before he attempts to take them? And why does this time vary from person to person?

The answer, of course, is the fear. It's been shown throughout the blogs - the more terrified people are taken first, some so utterly petrified that It can harvest them in our reality. On the flipside, those that couldn't be caused to fear It enough were instead forced to believe in It strongly enough for it to take them. Like Zeke.

The Game reaches a peak, and then The Reaping begins. Which brings me to my final point: Achieving Victory.

There are many ways It can win. It can scare a person until the manifestation is powerful enough for an on-the-spot attack. It can Turn them, such as with Andrew. Finally, though, it can Invite Them In.

These invitations are obvious traps. However, very rarely are they designed to heighten fear via a false sense of safety. To list those known so far:

Elizabeth's Diner.
Anthony's tree.
Tim's house.
Zeke's Warehouse.

Perhaps it's the dread of one facing It that solidifies Its existence. During The Chase, manifestations are random, weakening their effect. Come to expect one, and it could gain strength. Regardless, The Rules are immediately ignored after the invitation has been accepted, adding weight to the suspicion they are self-inflicted by It.

Finally is the process by which a person is Turned. It somehow begins clawing the contents of a mind out of a person, attacking their sanity and force of will. Eventually, all that is left is an obedient shell. The process of attacking a mind in itself suggests it is an element of the human consciousness, but it goes deeper. Numerous code phrases are used by those being Turned as part of their mental degredation and natural attempts to outdo an intruding presence. One such phrase is "it hurts," the phrase used throughout the Chzo mythos by those under the influence of either The Tall Man or other demonic entities intrinsically linked with it. If It had been attempting to influence Yahtzee to create him via Philip Effect, it may well have caused "it hurts" to make it into the game.

Proving this theory Pushing forward with this theory would mean countermeasures would revolve around forcing Imprints on It and defending against Its own imprints.