UNIX for Advanced & Expert Users

Hello,
In unix, the normal user can't create a hardlink for a directory. why..?
Is there any difference between creating a hard link for a file and directory?
The super user can create a hard link for directories.
Why we cannot create and super user can create.?

Please, can anyone help me........

A directory must have two special entries in it. A entry called . must exist and be a link to the directory itself. And an entry called .. must exist and be an link to the parent directory. The link system call does not perform these operations. In the old days, it took 3 link operations to create a directory. Now there is just a single mkdir system call to invoke. And root should be using mkdir, not link. Also by using silly combinations of link and unlink it is possible to break pieces of the file system off from the tree structure, create loops and even remove . or .. from a directory. I doubt that you could recover from
unlink /usr/bin
or something like that.