That is one of the most fruitful questions I've asked of this familiar Psalm.
As I explained about a year ago, the book of Psalms is not 150 prayers and hymns in random order. It's sometimes hard to know exactly what conclusions we should draw from the order the Psalms are in, but that they have been carefully arranged is beyond doubt.
Part of that arrangement is to divide them into 5 "books". Not one book of Psalms, then, but 5, where the numbering happens to continue consecutively through the whole volume. There appears to be some kind of flow through the 5 books. Predominantly book 1 (Psalms 1-41) deals with issue of King David's need to establish his throne in the face of many opponents. Predominantly book 5 (Psalms 107-150) seems to be set in the context of hope post-exile.
This makes Psalm 8 stand out like a sore thumb. Psalms 1-2 are introductory of the whole Psalter. But Psalms 3-7 and 9-14 are all about David's foes, and all about establishing his kingdom. Psalm 8 is a Psalm of praise to God's majesty, and it almost looks like it's been misplaced.
However, noticing this puts the wonder that man should rule the universe in its context. Psalm 8 draws on Genesis 1, where we are given the mandate to rule: Verse 8 of this Psalm quite consciously echoes Genesis 1:28 (“birds of the heavens”, “fish of the sea” and so on). Why should God make humanity the pinnacle of creation? But this is set amongst Psalms where David’s rule over Israel is struggling, and Psalm 8 is headed as another Psalm “of David”. So a Psalm about humanity’s rule is set in a section about David’s rule.
David’s rule then, the subject of Psalms 3-14 at least (arguably, Psalms 3-41), is a particular example, and even the pinnacle, of humanity’s mandate to rule. Humanity’s mandate finds its best expression in the rule of the king: The one who rules over the ruling race.
That makes Psalm 8 not just a marvel at the fact God should put humanity over creation, but also in context a marvel that God should put David over this race of rulers.
This is then where the way Hebrews 2 treats Psalm 8 comes in. The writer of Hebrews says that we do not presently see everything in subjection to “him”, but we do see “him who for a little while was made lower than the angels, namely Jesus, crowned with glory and honour because of the suffering of death, so that by the grace of God he might taste death for everyone.” Commentators differ as to whether the “him” to whom everything is not yet visibly subject is Jesus or humanity in general. Regardless, the seamless way in which the writer of Hebrews moves to talking about Jesus’ ruling looks confusing.
It looks confusing until we realise that Psalm 8 is celebrating the fact that David should be placed over humanity. It is then the most natural move in the world to shift to celebrating the fact that Jesus, as great David’s greater Son, should rule the world. We know God the Son rules, because he’s God, but for God to choose to have a human being in heaven on the throne is astonishing. In the light of 2 Samuel 7, this is exactly what Psalm 8 is marvelling at, so Hebrews 2 is exactly right.
This is where the interplay between humanity’s rule, and the rule of the anointed king, comes back into play. In seeing Jesus ruling the world, we are seeing God’s plan that humanity should do so, and that is wonderful. Equally, we read in Genesis of God’s plan for humanity to rule the world, and in Jesus this comes to fruition.