When a tooth has decayed beyond repair,  the last option for that tooth is to pull it out. Sometimes this happens because of periodontal problems. The tooth is mobile because the bone loss is so severe.  

Primary Teeth: (baby teeth)  As the permanent tooth is trying to push through, its job is to eat away the baby tooth roots so  the baby tooth can exfoliate and fall out.  In some occurrences the permanent tooth will not completely resorb through both roots and the baby tooth becomes stuck and cannot exfoliate on its own. The primary tooth may have to be extracted by the dentist if it does not come out naturally.