Not everything that seems like it could go in the dishwasher should go in the dishwasher, and a common mistake people make is putting sharp chopping knives in with your crockery and cutlery.
During a wash cycle the metal blades will get hot and wet, the heat and humidity can seriously damage both the steel (rusting) and the handle, especially if your knife has a wood handle.
Water jets in the dishwasher can knock the blade into whatever else is in there with it, which can damage the edge with brittle ceramic knives at an even greater risk.
In addition to all of this chemicals in the detergents can dull or discolour the blades.
Sharp knives can also scratch cups, plates, and even the machine if pointed down while pointing them up at you can pose an injury risk.
Therefore, when cleaning your sharp knives hand-washing in hot, soapy water will do just fine.