In To the Oak Tree, why did the author write about the love between two trees?

This is because the oak tree in the poem is not a concrete object, but an ideal lover symbol of the poet. Therefore, this poem, to some extent, does not simply pour out its passionate love, but expresses its ideal and belief in love through its closest and most concrete image, which is warm, sincere and loyal, which is quite meaningful to the ancients. I hope this answer is helpful to you.