The usual cause of a retained baby tooth is the absence of an adult tooth to replace it. Sometimes the adult tooth never develops, leaving the baby tooth in place while other adult teeth grow in around it.
For your dentist, the concern about your baby tooth is clinical in nature. Baby teeth can still fall out even after you reach adulthood. Without an adult tooth to take its place, this lost baby tooth leaves a gap that can cause problems with the function of your bite. Your adult teeth will tend to shift toward the gap, causing them to become crooked.
If it looks like your retained baby tooth isn't strong enough your dentist might recommend extracting it. After the removal the best option is a dental implant. Its a prosthetic tooth with a metal tooth root. Dental implants function just like natural teeth.
The most common aesthetic solution to make your baby tooth match the others is dental bonding. This is when the dentist uses a tooth-colored material to build-up the tooth so it is the appropriate size and shape. This material is permanently bonded to the baby tooth, allowing it to look natural and function normally.
There are a lot of considerations that go into deciding what should happen. Your dentist will do a full evaluation of your situation, including x-rays and help you decide on the best course of action.