Aside from “natural” causes relating to genetic inheritance, teeth can become misaligned due to environmental factors as well. Teeth, although they feel hard and unshakable, are actually anchored in what can be considered soft tissues, as well as bone. But teeth move about, and can be pushed into different positions, this is why orthodontics works at all. So environmental factors and the habits you have or had can affect how your teeth look.
The teeth of adolescents are very easily distorted from their natural growth patterns. As teeth are growing, the palate remains very soft up until the eruption of the second molars, when the palate will start to harden. Right after the second molars erupted is what can be considered the golden opportunity to receive orthodontics. This means that in the year after the second molars have erupted is the time you should get orthodontics. After that, certain things can only be done with great difficulty, including painful devices such as a palatal expander, the use of headgear, and you will probably need to wear a retainer indefinitely, to make sure that your teeth do not return to their previously set, pathological position.
Environmental factors that can change the alignment of your teeth is the practice of sucking your thumb or using a pacifier as a child. Trauma to the teeth and mouth can also be a reason for getting orthodontics later on in life. Repetitive stress to a certain part of the face can also change the way your teeth grow, and in which direction they do so, and can even change the position or angle that your teeth are in once they have fully erupted. The alveolar tissue remains soft, but hardens as you get older, so changing the alignment of teeth becomes harder too. I guess that is a mixed blessing.
What To Do
If your teeth have shifted from some sort of environmental factor, or from a lifestyle choice or habit, the first thing to do is to have a conversation and consultation session with an orthodontist. You will probably have to wear braces or a retainer or an aligner in order to rectify the situation, and once you are done, you will have to wear a retainer, possibly forever. But it is worth it in the end, as crooked teeth are one of the ways in which people will judge you, and they are less hygienic than straight, nicely aligned teeth, as those are easier to clean.