
Commencements, Middles and Endings is unavoidable sequence.
Lord Krishna suggests Arjuna not to mourn about Soul because he is unmanifested, unchanging, unborn and undying. He further explains by delivering verse-26,in which he tells that even if you think(because of confusion) this Soul has been born and he will be dying(which is actually not true), then also you should not mourn about this. Lord Krishna further clarifies in regards to consequences of birth and death in verse 27–28.
The Bhagavad Gita. Chapter-2, verse-27.
जातस्य हि ध्रुवो मृत्युर्ध्रुवं जन्म मृतस्य च |
तस्मादपरिहार्ये ऽर्थे न त्वं शोचितुमर्हसि ||
*Jaatasya hi dhruvo mrityur dhruvam janma mritasya cha;
Tasmaad aparihaarye’rthe na twam shochitum arhasi.*
Jaatasya- of the born. Hi- truly. Dhruvo- fixed.
Mrityur- death. Dhruvam- certain. Janma- birth.
Mritasya- of the dead. Cha- and. Tasmaad- from this.
Aparihaarye- in unavoidable. Arthe- in consequence.
Na- not. Tvam- thou. Shochitum- to mourn.
Arhasi- thou art worthy.
Translation:
Of the born truly death is fixed and certain are the birth of the dead. For this unavoidable in consequence, you are not worthy to mourn.
The Bhagavad Gita. Chapter-2, verse-28.
अव्यक्तादीनि भूतानि व्यक्तमध्यानि भारत |
अव्यक्तनिधनान्येव तत्र का परिदेवना ||
* Avyaktaadeeni bhootaani vyaktamadhyaani bhaarata;
Avyakta nidhanaanyeva tatra kaa paridevanaa.*
Avyakta- unmanifest. Aadeeni- beginnings. Bhootaani- beings.
Vyakta- manifest. Madhyaani- middles.
Bhaarata- Descendant of Bharata (i.e.Arjuna). Avyakta- unmanifest.
Nidhanaani- ends. Eva- again. Tatra- about this. Kaa- what?
Paridevanaa- complaint.
Translation:
Beings are unmanifested in beginnings, they manifest in there middles, O Arjuna. Again they unmanifest in ends. What to complaint about this?
Commentary:
Whatever born is bound to be dead and whatever dead is bound to be born again. These are inevitable movements which can’t be avoided. Therefore Lord Krishna suggests Arjuna, not to grieve. Every being passes through three stages that are commencements, middles and endings. Beginnings are unmanifested, middles are manifested and endings are again unmanifested. This sequence is fixed, so there is no reason to complaint about this.

