We refrain from committing them because we, as Christians, want walk the path that God has for us. You cannot earn heaven. Lets put it this way. No person could ever enter heaven with any sin....At all. Therefore, if all of your sins are not forgiven, heaven is empty. The price of sin is death, and Jesus paid that price. The only thing we have to do is believe and accept. Doing so will change you. It won't make you perfect, but it will make you want to be a better Christian just to be closer to God in that way.