One reason your teeth may hurt after fillings is that the tooth was overheated during the procedure and this damaged the nerve . To avoid overheating the teeth , the dentist needs to work slowly and use cold water periodically.Even if your tooth hurts after a filling , you should wait several months to see if the pain dies down , instead of immediately getting a root canal.
Another possibility is that the dentist used an anesthetic with a vasoconstrictor and therefore your tooth was deprived of blood flow which can not only damage the tooth , it can kill part of your jaw.
I think everyone has weaknesses in their enamel in some spots but I think you should'nt fill teeth that don't hurt.I have teeth I was told needed fillings years ago that I never filled and they don't bother me.
Needless to say , I don't trust dentists.I think you need to find a new dentist.