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.
The fall of mankind has introduced an upside-down situation, but God remains available and perfect (Gen. 3:14-24; Ecclesiastes 4).