Why does silver tarnish when exposed to sulfur or hydrogen sulfide?

Silver tarnishes when exposed to sulfur or hydrogen sulfide because it reacts with these substances to form a layer of silver sulfide ($A{g}_{2}S$) on its surface.1

While silver is highly valued for its resistance to atmospheric oxidation—meaning it does not react with dry oxygen or moist air—it is chemically reactive toward sulfur-containing compounds. When silver comes into contact with sulfur or hydrogen sulfide at room temperature, a chemical reaction occurs that creates this dark, dull tarnish. This process is a notable exception to silver's general stability, as it is one of the "noblest" or least reactive transition elements.12