Doors are perhaps - after walls and roofs - the most universal and utilitarian structural component of buildings. They can admit or deny access to people, animals, light and air. And, of course, being so common, their form is practically endless.
Beyond utility, doors have become decorative things. Functional components of doors wrought into elegant flourishes, retain their beauty even as they rust into decrepitude. Even something so mundane as a parking garage entrance attracts the creative attention of architects. For other doors, their original purpose has become almost secondary as they are transformed into artists canvasses.
But, utilitarian or decorative, or both, there is always a sense of mystery as to what lies beyond.