Because of years of learning and making blades of various designs and sharpening blades. What annoys me is when I see people putting the knife in the dish washer.
I know what you are thinking. Why would putting a knife in a dish washer annoy me. All it does is clean and sanitize it with hot soapy water.
Well It's that hot water that ruins the steel over time. How you ask. To answer that question you need to know the number one fact about knife making. No matter how good the knife looks or well it's constructed. The heart and soul of every knife is the heat treatment of the steel itself. Heat treatment is just a vague way of saying quenching and tempering processes.
It's these two processes that give it it's quality. As seen on the show Forge and Fire. Quenching where you super heat and quench it into usually oil. Which hardens the steel and makes it very brittle. The one thing you don't see in the show if they do it is the tempering. To temper you just use a lighter heat which is dependent of the steel. You let the steel heat up to a certain temperature for an hour in multiple cycles to soften the steel so it's not as brittle and retains it's hardness and edge.
Tempering is where our problem with the dish washer is. To temper stainless steel in the fastest way possible you temper around 300°f/149°c in three cycles. The hot water in the dish washer is at least 212°f/100°c. While this isn't as hot however this extreme temperature over multiple cycles through the dish washer will over temper the steel to the point the steel can't retain it's edge as long. Meaning you have to hone or sharpen the knife more often and if you are putting a new edge on more often. You are losing material faster and faster till the knife looks completely different.
If you want to maintain your favorite pocket knife and especially your favorite kitchen knives. Hand wash and dry them after every use and they will last for a very long time.