Why does the glass on my fireplace fog up when I turn it on?

There is a temperature difference between the air inside of the fireplace and the air in the room.  Warmer air has more moisture than cooler air.  When the fireplace is turned on, the temperature inside the fireplace begins to warm up and condensation forms on the cooler glass.  The water vapor in the warmer air is condensing on the cooler surface.   As the fire gets hotter, usually within a few minutes, the glass will clear.  Although it can be a bit annoying, this is not uncommon and really nothing to be concerned about.