Why do Endermen take blocks in normal world?

No one really knows, though there are theories. One of these theories is that Enderman were once normal humans, and the cure to return them back to humans is hidden underground. That is why they pick up blocks - to look under them for the cure. This also explains why they teleport - to cover more distance in their search. The reason they may want to be human again may be the fact that water hurts them, and they were so used to drinking water from when they were human. This theory also explains that Endermen are self-conscious, and think they are ugly as Endermen. That is also a reason why they want to be human. It is also why they attack you when you look at them.