Originally Posted by
Ted Striker
It's because the strings need to move through the nut. If the nut has grooves in it that prevent the strings from moving (aka the nut is worn out), or if the strings have enough corrosion on them that they resist moving, the guitar won't be in tune. Even if you tune it, the first time the string moves it'll sit in its new position and be out of tune again.
To prevent that from happening you should lubricate the nut with graphite (a regular pencil will work fine) or a guitar store will sell you some super expensive very light oil, same difference)