Because it is the beginning of the end of all that is bad in us.
Sorry if I explain stuff you already know, but when you become a Christian you dont automatically become perfect, but you allow Jesus to be part of your life and he starts helping you to know the truth about life andwhat is important and what is not.
I obviously have a long way to go still, see my longstanding argument with ...
anyway, Jesus died so our sins could be forgiven and so that would could be part of God's family.
It is very sad that it was necessary, but very good that it is done.
Jesus told his disciples repeatedly that he had to die, but no one believed him, which in a way is a good thing, because if they stopped him from givein up his life, we would not have that forgiveness.