It’s the protein.
Eggs stick to things when we cook them. Especially stainless steel, tin-lined copper and unlined aluminum. The liquid manages to get into even the smallest imperfections or indentations, then as it hardens when heated it sticks. This is worse the higher the heat.
I prefer to cook my eggs in cast iron. Well seasoned of course. I use a #8 Favorite Piqua ware skillet from Miami, OH. a little butter or olive oil is all it takes to cook an egg and have it release no problem off the bottom of the pan. I start on medium and get the pan (and fat) nice and warm, then I add the egg. Essentially the seasoning (the coating of hundreds of meals and the fat that has stuck to the iron) is a super smooth surface that the egg cannot get a hold of.
If you cook in a stainless pan, watch out. you will need more oil than you’re used to to coat the bottom of the pan and make sure that the egg can’t get in to the tiny little ridges from the pressing, or spinning of the pan during manufacturing. Keep the heat low to medium and if you’re careful the egg won’t stick.
Then there’s non-stick. It comes in a variety of types, but they are all designed to do the same thing: keep food from sticking while cooking. A side effect of this is that less oil or fat is needed to cook the food, but keep in mind that if you’re trying to really brown or sear something and develop a fond (the brown flavorful outside) you can’t.
Non-stick is GREAT for eggs. It’s the only thing I use it for. I know I said I cook my eggs in cast iron, but sometimes the potatoes are cooking in the cast iron, so I reach for the non-stick.
A small 8″ pan will do one sunnyside up perfectly. you still need a little oil (olive is my preferred here since you don’t need to go crazy on the heat) and you cook it until you like it.
Over easy for me.