Knives get dull because the edge folds, not, as popularly believed, because the edge wears away and gets blunt.
• There’s no such thing as a "never-needs-sharpening" knife. So-called "never-needs-sharpening" knives are serrated. Serrated knives DO get dull.
The tips of the serrations fold.
To delay dulling, a sharp edge must have two main characteristics:
• SYMMETRICAL: A perfectly straight, symmetrical edge does not tend to lean in one direction or the other and thus does not fold as easily.
• ARCH-SHAPED: An edge with more metal behind it resists folding longer than a thin, weak "V" – shaped or hollow-ground edge.
Sharpening steels (and other devices which include rods) straighten the edge vs. actually resharpening it (creating a fresh, new edge). They take skill and effort to use. The unfolded edge is still weak and will immediately start to fold again. Eventually the edge is too weak to strengthen and the knife must be completely resharpened.
Manual sharpeners with washers/discs lack reliable angle control and sharpen by skiving off pieces of the edge.
Sharpening stones resharpen the edge. They take uncommon skill to hold the correct sharpening angles. It takes time and effort to use a stone.
Sharpening services resharpen the edge. Many use inexpensive grinding wheels that overheat (detemper) knife edges and take off too much metal. Excess heat weakens the blade so it does not sharpen as well or stay sharp as long. Some professional services are more skilled than others.
Electric sharpeners resharpen the edge. They sharpen safely and effectively and put stronger, longer-lasting arch-shaped edge on the knife.