The two parties have irreconcilable differences of values at the core: the role of government, the nature of abortion, secular vs. spiritual influences in education and daily life, the meaning of the Constitution, to name a few. These will always inform the debate between most democrats and republicans and will naturally cause proponents on both sides to argue with passion (and lack of reason) about minor but sensational details.
You might as well ask why there has always been war.
The two parties have irreconcilable differences of values at the core: the role of government, the nature of abortion, secular vs. spiritual influences in education and daily life, the meaning of the Constitution, to name a few. The differences between the parties, as such, are a lot blurrier than the differences between conservatives and liberals (without getting into shades of fiscal/social conservativism/liberalism). Personally, I'd like to see both major parties disband and have everyone realign along lines of what their convictions actually are, with new names to avoid the baggage of connotations no longer applicable.