First, the endometrial cells are super sticky, so they stick to each other, and stick to other tissues abnormally. That’s why clotty periods are common. But it also means that when the cells go into the body cavity, they stick to things they shouldn’t and act like glue, which means that other body tissues start adhering, too.
Second, endo cells secret chemical signals that tell the immune system that they are supposed to be there, so the immune cells don’t clean them up the way they normally would. That means the endo cells stay there.
Third, endometriosis cells don’t die the way they should. Most normal cells undergo a process called apoptosis, a process of normal cell death. Endometriosis cells have the kill switch off, which means they stay alive and divide, creating more of themselves.
In short, endometriosis cells go where they don’t belong, stick to things they shouldn’t, don’t get cleaned up when they should, and don’t die like they ought to.