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Why does Endermen not has grammar?
Other than that, the most likely explanation as to why they so despise being looked at is the fact that they feel mocked by the blissful inhabitants of their former home world.
If you think about the various ruins scattered about the world, and the fact that Endermen even now have a propensity to pick up blocks and place them where they see fit, as well as the fact that the Stronghold, yet another ruin, is the access point to the home of the Endermen, a picture begins to form of an ancient race that constructed a vast empire.
However, their curiosity and hunger for power sought too far, and they attempted to breach even the realm of the gods themselves. What resulted was the creation of a portal to the world we now know as the End, Endermen being the twisted, tormented forms of those whose hubris took them too far. They were corrupted by the energies of the End as punishment for their crimes against the gods of the Minecraft world, and are forced to blink in and out of existence whenever they walk the lands of their former world. What we see as teleportation is truly the Endermen flitting back and forth between the overworld and the End without control.
Forever banished to live an ethereal existance, never again able to have a permanent hold on the world they once charished so, the Endermen now hold a deep hatred for those of the new world, and unleash that hatred on them when they feel the stare of those who have taken what was once their empire.