Your teeth support each other like beams of a house. When one beam goes down, it can cause changes all throughout the house. When you lose a tooth, similar changes occur. Without replacing a missing tooth, there will be a large gap between your remaining teeth. As a response, your remaining teeth will begin to move into the empty space. This can cause changes in your bite as well as misaligned teeth. As a result of the new misalignment, you may also develop a TMJ disorder too.
Lastly, with your newly misaligned teeth, you will also find it more difficult to effectively clean the teeth. This can result in tooth decay and gum disease.