Surely only God can forgive sins; why do I need to tell them to a priest?
It is true that all forgiveness comes from God alone, which is mediated to us through the cross of Christ. However, sometimes we can be rather lazy in our self examination. By making a list of sins and then by going and confessing them to a priest, we a forced to confront ourselves and to see how far short we have fallen from God's ideals. This can sometimes be a painful and even embarrassing process, as we feel bad; but that is not a bad thing, since many healing processes can often be painful. We all know that by identifying the problem, you are half way to its solution.