Richard J. Staller, D.D.S.
There are several possible reasons for it, including:
1. Contamination (from saliva, blood, or other debris) of filling/tooth area during the filling procedure
2. Poor-quality filling material or insufficient light-curing of filling material
3. The filling is too large (pushing beyond the physical limitations of normal filling materials), where a partial or full-coverage crown may be required instead.
4. Your bite or grinding/clenching habits, if any, can seriously affect filling retention and wear
5. Your diet: acid erosion - most often from dietary acidic foods and drinks, but sometimes from acid reflux (from stomach). This is a growing trend unfortunately. I have many patients suffering from acid reflux and/or have highly-acidic diets, and as a result, I sometimes have to replace a filling occasionally because the margins of tooth that initially supported the filling have dissolved away.
So... ask your dentist what can be done to ensure filling stays put. If you don't get a good explanation, then you may want to find another dentist - at least for a 2nd opinion. Obviously this depends on your relationship and trust in your current dentist.
I hope your haven't been charged for them each time. I always provide a limited warranty for my fillings, to all of my patients. Ask your current dentist about that.
Hope this helped.