What's in a tooth?  There is no caramel center covered with chocolate, but a pulp made of nerve tissue and blood vessels surround by dentin, cementum and enamel.  Enamel is hard; really hard but it can be attacked and slowly destroyed.  We call the 'getting a cavity'.  If the decay gets big enough and goes deep enough it can infect the nerve and cause it do deteriorate.  When this happens a root canal procedure may be necessary and then a crown.  After all of that however the tooth is NOT as good as new.  We cannot make a tooth as good as new.  So, it is best to avoid and prevent decay.  If that is not possible just understand that any treatment may last a long time, but it may not last forever.