Here's why:

* Metallic bonding: Metals have a unique type of bonding where electrons are delocalized and free to move throughout the structure. This allows for interaction with light in a way that causes reflection and creates the characteristic shine.

* Surface smoothness: Many metals have a smooth surface that contributes to their luster. This allows for light to reflect evenly, producing a bright, shiny appearance.

Examples:

* Gold, silver, and copper are known for their bright, metallic luster.

* Iron and steel also have a shiny luster, although it can be duller depending on surface treatments.

However, it's important to note that:

* Luster can vary: The luster of a metal can be affected by factors like surface oxidation, impurities, and the specific type of metal. For example, aluminum has a duller luster than silver.

* Not all metals are shiny: Some metals, like lead, have a relatively dull luster.

Overall, while luster is a common characteristic of metals, it's not a universal property. Still, it's a strong indicator of a metallic substance.