Americans have some wonderful smiles, of which, some of them, come quite naturally to a given individual, whereas others are created by the aid and the usage of orthodontics. Quite obviously, for very young children that get orthodontics, such as preteens, of which, their permanent teeth have not yet all come in, they are in all likelihood, going to need additional orthodontic treatment after those adult teeth do come in; of which, orthodontics at such a young age is typically recommended to take care of an overbite or to basically help preclude current problems from getting worse by addressing them at a young age.
Then there are all those others that get their braces after their permanent adult teeth have come in, of which, such are typically teens, but many are adults, as well as even older adults, in which, their orthodontist comes up with a plan to create that winning smile, that Americans so hanker for. After the orthodontics phase of straightening out the teeth with braces is accomplished, a retainer is then created for those teeth, which is meant to be worn at night, or as necessary, in order to keep the teeth straight and in their appropriate position.
The main problem with retainers for some people, is that they don't follow the instructions as given by their orthodontist for the usage of their retainer; in addition to the fact that most users of braces, after completing their treatment, do not ever see their orthodontist again. As might be expected, not following instructions has consequences, of which, the very first one is the basic non-recognition by patients of how teeth are moved via braces to begin with. That is to say, the whole reason why teeth will move through orthodontics is because of the pressure applied, not to the tooth itself, although it may seem that way, but actually to the ligaments that hold that tooth in place. Therefore, not too surprisingly, in absence of that pressure still being applied, through the regular and the correct usage of that retainer, and especially the amount of time that the retainer is actually on in a given day, then the teeth having no longer any pressure being so applied that will preclude them from moving, will ever so slowly, but inexorably, go back to where the ligaments feel comfortable having them be placed.
This means that over a period of time, someone that has had braces, even for a lengthy period of time, may look in the mirror, some years down the road, and clearly notice that their teeth and therefore their smile is no longer straight but that their teeth are actually back to being crooked, necessitating getting orthodontics to correct such, once again.
The prevailing reason that some people need orthodontic treatment more than once, is very similar to someone that has corrected a former bad habit with a good habit, but inexorably, have gone back to their bad habit; often because of a lack of concentration, diligence, and preventive maintenance; believing incorrectly, that they no longer need to be on watch, when in actuality there are certain things in life that require eternal vigilence.