Metals are shiny because metals contain free electrons that vibrate when they come in contact with light. When the electrons vibrate, they produce their own light. This is reflected back and is what creates the shiny, lustrous appearance of metal.
Metal is shiniest when it is first cut. Exposure to elements such as oxygen and carbon cause metals to lose the luster on the surface because the elements cause a tarnish that reduces the freedom of the electrons to vibrate. The tarnish is removable through polishing or using a chemical treatment. Some metals, those known as "noble metals," are more resistant to tarnishing caused by elements.