Ok now you know the story of the Enderpearls listen to this....

Sometime ago people built lots of old fashioned houses but they were forced to live in the nether because of the monsters. The monsters needed away to protect the Endermen so they used a power so powerful it could destroy the whole of minecraft. It was called silver lead making millions of fish known as Silverfish. Since a human got into the end before, the Silverfish gaurded it. They also gaurded other precious bricks, if any human was to mine those bricks.... it would be the end for them

And thats why the End portal is always gaurded with Silverfish