The 16 expressions of forgiveness

The parable from Shoftim of the trees looking for a king, and choosing the thorn bush, as an allegory for the divine service man, who is a "tree in the field."

This parallel, brought in Akeda, suggests people who are looking to throw off the yoke of heaven. Fruit trees are those people with desirable traits, "I found you as an apple amongst the forest trees." For this reason you can only cut down trees that don't bare fruit, compared to the wicked ("I will cut off the enemies of Hashem" etc).  They were therefore only satisfied when they crowned them the thorn. 

The Tzemach Tzedek compares looking for this "old foolish king" of the thorn, the evil inclination, with the building of the Tower of Bavel. The statement, "let us make for ourselves a name" is this concept of faux haughtiness to derive divine life undeserved. To reach up and take from the top (like from the cookie jar on top fridge). On this it says, if you lift yourself up like an eagle, from there I will bring you down." 

The concept of how the fruit trees did not want to "abandon their fruit" and "wave over the trees is that this "waving" is a concept of joy. However, being that it is without fruit, the fruit of the righteous being mitzvos, this joy is without "simcha shel mitzva." Rather, as expressed by Yishaya, "their drunken wine parties, with drum and flute... they do not notice the acts of G-d." This fruit is rather a "gefen nochria," a foreign grapevine, just as "kela ilan" is like a foreign "tree" to techelet.

So the thorn says, "if you're desire for the mundane actually has a higher purpose, to elevate what's below, you will find shade in it, an encompassing light. Just as the Levona in the ketoret is the the eleventh level, a makif to elevate the inanimate spices, so to involvement in the world to elevate the "klipas noga" can bring a purposeful shade to the thorn.

If, however, you have bad intentions, "a fire will start from the thorn and burn down the cedars of Lebanon." The tall trees, like the Tower of Bavel and the eagle, that reach upwards to nuture from the divine in their haughtiness, will be "burnt up" with the three impure klipa. ("I will wipe away the spirit of impurity from the earth" and "...will go up to judge mount of Eisav" etc.)

From Ohr Hatorah Shemos Ubagefen Shelosha Sarigin