In the Gītā, Kṛṣṇa says that in this material world death is inevitable. Everyone who is born is sure to die according to their allotted karma, so ultimately there is no need to grieve. Read about life, death and re-birth in Bhagavad-gītā Chapter 2 – Sāṅkhya Yoga (The Yoga of Analysis) – Verses 13-15 and the Anuvṛtti