This is my opinion or thoughts, so hope you enjoy this :)

Before starting, I want to talk about Minecraft 1.9 and 1.10 update. These two updates had add many features like End Crystal, Elytra, Dragon Head, etc. Thus, they also add in Enderman's apperance in the Nether, that makes Enderman the only mob to be naturally existed in all three dimensions. Don't you feel weird why Enderman goes everwhere? We will leave it for later and move on to the End Crystal first.

As most of you may know, End Crystals are things that regen Ender Dragon's health, also you can summon the Ender Dragon mutiple times by them and they are there when you come to the End. Additionally, End Crystal can now be crafted, for those who don't know the recipe, it is very simple to remember: glasses, eye of ender, ghast tear. Wait...ghast tear? There is no ghasts in the End, so are glasses. Did someone collected them? Someone who has the ability to go to dimensions without portals? Yep, you already guess that it is Enderman. So there you are, the reason why Enderman travel around: to take ingredients for End Crystals, glasses from the Overworld, ghast tear from the Nether.

Then back to the new feature of 1.9, summoning Ender Dragon by End Crystals. This feature reveals a major thing about Ender Dragon...When the Enderman found all the ingredients and crafted End Crystals, the only reason why they crafted them is to summon in the dragon. Maybe it's a legend creature of the End... That means the existance of the dragon is because of Enderman. So there you go, the Ender Dragon, or should I say an Ender Dragon couldn't be there unless for the Enderman. However, the reason why they summon it in hadn't been revealed yet.

That's all guys, thank you for reading. You may get confuse why the Enderman made the End Crystal first, not the old players. It is because you need eye of ender to be able to go to the end, by killing Enderman and when you get there, the End Crystal are already positioned and the dragon is right at your face :) Hope I cleared that out for you, thank you for reading :D Sorry for the arrticle being a bit clumpsy, English isn't my first language :D