Why does God allow controversy?

Christians arguing drives me mad. That reaction is partly righteous because unity brings blessing, and partly unrighteous because I can be tempted to think that everyone should agree with me. Given how inexperienced and immature I am this is an awfully ridiculous thing to think.

These words from Wayne Grudem in response to a question about why God allows controversy are hugely encouraging, and a rebuke to my simplemindedness:

Well, for one He wants to test our hearts and see what our attitude is towards those with which we disagree. And two, He purifies the church through controversy because our positions are then deepened and strengthened.