If I don't wear my retainers for a couple of days my teeth start to move. My friend never wears her retainers and her teeth have not moved. Why is that?

Orthodontists and researchers have tried for years to explain this stability issue and the answer remains the same. We cannot predict with any degree of certainty how stable a person's result with be over a long period of time. We can say that if you wear your retainer for a certain amount of time, your result will be pretty much remain stable. Our bodies are constantly changing, and the forces on teeth are constantly changing, which cause teeth to shift. Teeth shift even if you never wore braces. Orthodontic patients should always wear retainers as instructed to ensure the most stable result possible.