In Hebrews 2:9, why does God make Jesus a little lower than the angels? As God's Son, he had no sins -- right? How should I understand this?

Originally Psalm 8, which is quoted by the Hebrew writer, is referring to humans, not Christ. Then it was applied to Jesus Christ, as the perfect, paradigmatic, and ideal human. Hebrews 2:6-9 reads,

It has been testified somewhere,

“What is man, that you are mindful of him,
    or the son of man, that you care for him?
You made him for a little while lower than the angels;
    you have crowned him with glory and honor,
putting everything in subjection under his feet.”

Now in putting everything in subjection to him, he left nothing outside his control. At present, we do not yet see everything in subjection to him. But we see him who for a little while was made lower than the angels, namely Jesus, crowned with glory and honor because of the suffering of death, so that by the grace of God he might taste death for everyone.

When Jesus became human, he shared in our humanity — an important sub-theme in Hebrews — he lowered himself. Notice the adverbial phrase in v.7: "for a little while." This is the doctrine of the incarnation. As in Philippians 2:6-7. Jesus was lower than the angels only temporarily (about 35 years). Then he returned to the right hand of God.