I would argue that the right answer is the first one: It elevates the relationship between the boy and the horse, suggesting that it is special and unique and private. D. H. Lawrence is well-known for his poems revolving around the spiritual life of plants and animals. In this one in particular, he is bringing together a young person and a white horse, both pure creatures that, precisely because of their natures, understand each other and communicate without words, reaching a degree of trust and rapport that transports them to another, more intimate and private, world.
