Why do we use wine or grape juice in kiddush and havdala?
God is perfect, and (as opposed to Christian and Moslem thought) all creation is good when used in the proper time, place and proportion. We are meant to rejoice on Shabbat and holidays, and a little wine helps us get into the proper mood. Similarly, we want to enter the new week on the right foot, so we use wine for havdala, as well. On the other hand, some people don't enjoy wine or can't handle it medically, so the halacha enables grape juice for them, which fulfills the ceremony of wine but without the "side effects", when necessary.