The harder a cutting surface, the more quickly a knife dulls. Hard surfaces include metal, marble, china, crockery, enamel, glass, and most kitchen countertops. The softest, and therefore the most desirable of the popular cutting surfaces, is wood. Though softwood does less harm to the knife's edge, hardwood is used most often because it absorbs less moisture and lasts longer.
Polyethylene boards are not as hard as, say, metal and glass, but they are harder than wood. Consequently, a knife becomes duller faster on polyethylene boards than on wood ones. Even though polyethylene is easier to clean, most good cooks insist on wood cutting boards because keeping a knife sharp is crucial.
Hard cutting surfaces are not the only anathema to a sharp knife blade. A blade that nicks too many bones or scrapes hard kitchenware in a dishwasher or on a drying rack also may not cut the mustard.