Most front and back doors on houses these days will more often then not have a glass panel. This maybe one large one or several smaller panes of glass. The reason that doors have windows is to let as much light as possible in. Corridors and hallways that lead into a home can be dark so having glass in the door can help to brighten them up. When you think about replacing glass windows you automatically think that it would be the normal window at the back and front of the property, but in actual fact the windows on the doors would have to be replaced more regularly.
It can be very common for the glass in a door to break quite easily and often more then once. The glass can become weak and have more movement if the door is slammed. It can cause the glass window to smash if done consistently. Closing the door softly can help prevent and weakening of the glass. You will see that single pane glass in wooden doors are more prone to smashing then UPVC doors. These kinds of doors will have double glazed windows more likely in the design on the normal windows.
Replacing windows in your doors would be a job for an expert as it can be a tricky job. When dealing with a door you need to be extra careful not to damage it any further. Front and back doors can be very expensive and if damaged to much you may need to replace the whole door. Often when the surround around the glass window is damaged it is harder to insert the new pane of glass. Fitting a new pane of glass to a wooden door is much easier as you have a larger area to go wrong. A wooden door can be filled and sanded if you made a mistake with measurements or such like.