Why does it say that my disc is dirty on xbox 360 when it is perfectly new?

The same thing happened with my brand new Gears of War 2 disk. All i did for it to work was use a disk cleaner and restarted my XBox. Even though a disk may be brand new and scrath free fingerprints and the oils from our hands can interfere with the reading of the disks. This is why i suggest to clean your games ever so often to keep them not only protected and scratch free, but fingerprint free as well so they can be read by the system with no problem.