The twenty verses give different reasons for not grieving, for different listeners, and they agree in outcome. One is the lasting self (2.16–25), one is the unavoidable cycle (2.26–28), and one is the self in every being (2.29–30).
Arjuna asks who knows yoga best. The Lord answers in terms of being joined to Him, not of knowing: the one who fixes the mind on Him, stays steady and has supreme faith is the most united (12.2).