Why did my tile, stone or grout become so dingy in the first place...it's only a year old?
Rarely do installers seal the installation. Why? Sealing is generally not included in the installation price; it would be at an additional cost. It requires yet another trip back to the job after the grout has cured for several days. Unless it is specifically asked for, it won't be included.

How often do you have to reseal the grout after first application?
Well sealed stone or grout can last for a few years IF the right cleaners are used. Most grocery store cleaners are acid based, and will actually remove the sealer. A PH-neutral tile & stone cleaner is needed for proper maintenance. When cleaning it's important to change your cleaning-water when it appears dirty. Always use a mop to agitate cleaning agent on grout surface. Dry grout with towel or terry cloth.