וְאִם-לֹא תוֹרִישׁוּ אֶת-יֹשְׁבֵי הָאָרֶץ, מִפְּנֵיכֶם--וְהָיָה אֲשֶׁר תּוֹתִירוּ מֵהֶם, לְשִׂכִּים בְּעֵינֵיכֶם וְלִצְנִינִם בְּצִדֵּיכֶם; וְצָרְרוּ אֶתְכֶם--עַל-הָאָרֶץ, אֲשֶׁר אַתֶּם יֹשְׁבִים בָּהּ
But if ye will not drive out the inhabitants of the land from before you, then shall those that ye let remain of them be as thorns in your eyes, and as pricks in your sides, and they shall harass you in the land wherein ye dwell ... Bamidbar 33:55
So, why does the Lord really fear the Canaanites? Let me tell you, for the answer hides in His words.
As Thorns - לְשִׂכִּים
The Hebrew word typically translated as "as thorns" (namely, לְשִׂכִּים) derives from the shoresh (magical 3-letter root) שכך which means "to calm down." The war-mongering Lord fears that the Canaanites will civilize His obediently bloodthirsty Israelites. Under the "evil" influence of the Canaanites, the Israelites will become recalcitrant to wage the Lord's bloody wars, and instead will seek to return peacefulness to the Land.
As Pricks - וְלִצְנִינִם
The Hebrew word typically translated as "as pricks" (namely, וְלִצְנִינִם) derives from the shoresh צנן which means "to chill", "to cool" and "to shield with a protecting cool-headedness." The hot-headed Lord fears that the Canaanites will influence His easily emotionally-driven Israelites away from violent barbarity and toward civilized cool-headed thoughtful, civilized rational reason. He fears His people will "chill out" and prefer to have barBeques with the neighbors instead of killing them.
And Harass - וְצָרְרוּ
The Hebrew word typically translated as "and harass" (namely, וְצָרְרוּ) derives from the shoresh צרר which means "to bind together" and "joining." Most of all, the fearful Lord fears that His stiff-necked Israelites will find that they actually like the Canaanites, will form mutual kinship bonds with the Lady's Pagans, and will join and bind together to "calm down and chill out" the fearful Lord Himself.
Taken together, we can easily understand then that the Lord fears the power of the Canaanites who follow Goddess (also called the Lady in Pagan Witchcraft) to successfully transform Him in the very heart of and throughout the Land. He fears the Lady and Her Canaanites will civilize Him and all His Israelites. She and Her Canaanites are just so wicked, don't you know!
You see, the Lord just cannot win a war of words with the Lady, He fears. LOL.