The fact of the matter is that no artificial material can adequately perform the job of your natural teeth.  Constant pressure exerted on our teeth from chewing, grinding and biting down on hard foods wears down on fillings.  The fact of the matter is that all fillings will eventually wear away, crack, become chipped or fall out.  The average lifespan of a filling is 5-15 years (it is said that fillings last even less time when they are white). 

Once some sort of damage is sustained to the tooth or its filling bacteria can form.  Since you can't clean inside the cracks of a filing, the bacteria feed on the food debris and form the acid that causes tooth decay. Decay under a filling can escalate quickly without you noticing any pain or sensitivity.

These problems can be prevented by having your fillings checked regularly and by getting your fillings replaced when problems are found.  Listed below are even more reasons and benefits for having old fillings replaced.

Leaking
When fillings become worn around the edges or pull away from the tooth enamel it is called leaking.  Leaking allows for bacteria to enter and be stored in the mouth in the inaccessible leaks that are in the edges around the filling.  Damage caused from leaking often goes unnoticed because there are no noticeable symptoms present until this decay reaches the nerve of the tooth.
When the damage does reach the nerve of the tooth it often causes severe decay and pain.  Obviously, it is important to keep the tooth and filling clean so as to prevent the development of another cavity. Worn fillings need to be restored as soon as possible in order to stop the decay from forming and the bacteria from spreading.

Biting and chewing
Cracks can develop in fillings simply from everyday biting and chewing forces.  The average human jaw has the ability to exert hundreds of pounds of pressure!  Chewing on particularly hard foods can magnify the problem.  If for no reason but simple chewing your food, your fillings can become worn, cracked and in need of replacement.

Crowns
When a tooth has had repeated trauma and a simple filling is no longer sufficient to protect and give strength to the tooth a crown is used.  Crowns are used to protect especially weak teeth from fractures and/or breakage

Looks can be deceiving
Damaged teeth are not always apparent.  Although decay can discolor a tooth, often times problems with fillings can exist without any apparent signs or symptoms.  Your dentist has the tools to be able to examine and evaluate the health of your tooth enamel.  In fact, dentists have a cavity detecting dye that can be rinsed over the mouth.  This dye sticks to areas that are decayed and rinses clean from areas that are healthy.

X-rays
More evidence for the benefits of replacing old fillings can be seen from X-rays taken to see decay that doesn't show on the surface.

Grinding or abnormal force
Cracked, broken, or fillings that are worn from unusual use will need to be replaced.  Activities such as nail-biting, tooth grinding (bruxism) or using your teeth to open things, will make filling damage more likely and a damaged filling can allow food debris and bacteria in your mouth to seep down and cause damage to the tooth itself.