Why do we try to have separation of Church and State?

To keep religious zealots from imposing their religious beliefs on others. Freedom of religion also means freedom from religion. Just remember that others may have different beliefs than you, do you want those beliefs imposed on you? I suspect not so why do you think you should have the right to impose your beliefs on others? You want Christmas trees? Someone else might want an inverted pentagram or an upside down cross. Can YOU live with that?