The temporal nature of things "under the sun," and newness only being found in Christ (Eccl. 1:1-18, Eph. 2:5-6).
Solomon shares the repentant reflections of his own life, recounting his failure to find meaning in anything.
Human beings are a vessel made to contain God (Ecclesiastes 2:1-17, 3:11; Romans 9:23, Ephesians 3:14-19).