hard hearts can be defined as unforgiving or just plain selfishness. today as in Jesus' time on earth people were not willing to work things out because that takes work.

It is my understanding that duet 24:1-4 is a consession and it shows finite love and grace for fallen man. This has been used today to justify divorce. When we follow Jesus and read the bible we start to understand how much God loves us and forgives us. When we understand what has been done for us we are more apt to forgive someones wrong doing and thus, not have our hearts hardened.