Why is this?
One reason the debate rages is that both sides can easily point to bible passages that seem to support what they believe. For every Ephesians 1:3-5 there is an Acts 17:30-31. For every Romans 8:29-30 there is a John 3:16-18. For every Acts 13:48 there is a Romans 10:12-14. And on and on it goes.
Another reason the predestination argument continues is that the two sides (and unfortunately it has become "sides") believe the other side is grossly misrepresenting the character of God. For example, those who emphasize God's sovereignty often portray the other side as describing a weak, powerless God. Conversely, those who emphasize the free will of man say that the other side believes in an unloving, unmerciful God. Both sides frequently employ straw man arguments.
Finally, the two sides usually refuse to actually communicate, instead talking past each other. This only increases the problems.
What can we do about this? For one thing we must realize that this should not be a cause of division. Jesus has made it clear that He expects His body to be united. Christ never made a claim that we have to believe a certain definition of predestination.
Second, let's actually communicate. When we do, we often find that on issues like this we agree on much more than we think at first.
Third, let's avoid straw man arguments. They are unhelpful and only lead to increased division.
Fourth, we may have to agree-to-disagree. This does not mean that we have "caved in" on what we believe, but rather that we will hold to the unity of the body in spite of our differences.
Fifth, we must avoid taking sides. When we avoid this, we make unity much more of a possibility.
Like so many other doctrines, predestination should never be an excuse for disunity in the body of Christ. We can hold to different definitions but remain united in Christ. This can and should be a real unity that includes fellowship as opposed to simply some sort of theoretical "unity in spirit."
Predestination has the potential to be divisive if we are determined to win an argument. It also has the potential to unify the body by teaching us how to live together despite our differences.