It is stainless ... not stain proof.
Discoloration can be caused by ingredients found in tea, coffee, salad dressing, vinegar and salt. Pitting or spotting are usually caused by hard water or from foods with high salt content.
Do not use steel wool or steel brushes.
These products leave little particles in the surface of the steel and inevitably these particles begin rusting and staining the surface of the steel. They also can excessively scratch the surface of your stainless steel. Stay completely away from steel wool and steel brushes.
Do not assume it's the cleaner.
Use a good quality stainless cleaner to remove the "build up" caused by minerals
If you do have some spotting or staining, and you've followed all of the rules, it may not be the stainless steel cleaner. Water, especially hard water, can leave spotting and staining on stainless steel surfaces. Towel dry after rinsing can end the problem. Sometimes, even water is an enemy to stainless steel.