Perhaps it is better said that the psalm is odd, but the choice of the verses was not. After all, the Christians were anxious to make sense of their Jesus, and when they found in their scriptures notions that spoke to them, that made plain to them what they had witnessed, or that much later helped them to understand what their ancestors said that they had witnessed, they grabbed the verses and used them to clarify and exegete the events that had become central in their lives. There can be no problem concerning using texts in this way as sources of enlightenment and clarifying power.
The problem arises, as I have said many times, when Christians take the next, erroneous, step, and claim that Psalm 118 is a prediction, a prefiguring, of the Palm Sunday donkey ride. That plainly will not do, for one simple reason: Psalm 118 then has no meaning for the ones who composed it! That is not only absurd, but monumentally arrogant. Certainly the poet of Psalm 118, and those who sang the poem in Temple and sing it still in synagogue and home, composed the piece with some purpose in mind for his/her own worshippers. And I think that purpose is clear and precisely what I have been saying from the beginning of this article. Psalm 118 is a theological summary of how YHWH relates to YHWH's people, as well as a summons to those people to praise the YHWH who creates and saves them and will continue to love them into all the future there is for them. "You are my God, and I will give thanks to you; you are my God and I will extol you" (Ps. 118:28). The psalm tells us why we should extol God, and why we "shall recount the deeds of YHWH" (Ps. 118:17). The reason? "God's chesed endures forever." That is all we need to know, and that fact underlies all dealings with this God, even when the one we call Christ rides into Jerusalem to his terrible death and his glorious victory over death.