Live and Let Live

Why is it that people tend to fight so often over religion? Why not ‘live and let live?’
It seems like such an easy concept. Simply live and let live. Ignore those who do not agree with you and they will do the same; if you do not harm them they won't harm you. Even so, we see countless cases of religions colliding throughout history, and even today, ending in very violent and deadly events. You would think that the same premise of naturalists and supernaturalist being able to set aside their differences and work together would apply in the same manner. Instead it would seem that when one religious group meets any other group that goes against their beliefs, they can not simply turn away. It is a problem that plagues out history. Perhaps it is because science can answer some questions that religion tends to dismiss and there are some questions that religion can give a more satisfying answer, like why people are killed in such horrific manners. However, when two different religions try to answer the same question they are often very different views, like different 'gods' or 'heavens.' In a way it acts as a much more direct attack on what a religion believes.