Consider when you are in a particularly grouchy mood, when things are not going well. Who is the most obnoxious person to have around? Someone who is in a good mood. There is no one more annoying to a grouch than the sunny optimist.
If you have been stuck for two weeks in the depths of a cave, what would be most tortuous to you? Light.
If you have been swinging a golf club incorrectly for many years, who is most painful to you? The teacher who compels you to change everything you've known and to swing in a new way.
The world, on the biblical reading, is a dysfunctional family. As G.K. Chesterton put it, "We're all in the same boat, and we're all seasick." Therefore in light of this disorder, when Jesus comes, he necessarily comes as a trouble-maker, as a breaker of the peace.
It's helpful to remember during Advent that there is no contradiction between God's gentleness and God's fierceness: they are both expressions of his love. They both emerge when love breaks into our dysfunctional world and sets it right, which is precisely what commences at Christmas.