Bhagavad Gita 2.27

jātasya hi dhruvo mṛtyur
dhruvaḿ janma mṛtasya ca
tasmād aparihārye ‘rthe
na tvaḿ śocitum arhasi

Translation of Bhagavad Gita 2.27

One who has taken his birth is sure to die, and after death one is sure to take birth again. Therefore, in the unavoidable discharge of your duty, you should not lament.

Commentary by Sri A.C. Bhaktivedanta Swami Prabhupada of Gaudiya Sampradaya:

One has to take birth according to one’s activities of life. And after finishing one term of activities, one has to die to take birth for the next. In this way one is going through one cycle of birth and death after another without liberation. This cycle of birth and death does not, however, support unnecessary murder, slaughter and war. But at the same time, violence and war are inevitable factors in human society for keeping law and order.

The Battle of Kurukshetra, being the will of the Supreme, was an inevitable event, and to fight for the right cause is the duty of a kshatriya. Why should he be afraid of or aggrieved at the death of his relatives since he was discharging his proper duty? He did not deserve to break the law, thereby becoming subjected to the reactions of sinful acts, of which he was so afraid. By avoiding the discharge of his proper duty, he would not be able to stop the death of his relatives, and he would be degraded due to his selection of the wrong path of action.Commentary by Sri Vishvanatha Chakravarthi Thakur of Gaudiya Sampradaya:

Since (hi) death is certain (dhruvah) with the depletion of karmas meant for this life, and birth is also certain because of the actions done by that body which just died, since both death and birth are unavoidable, do not lament.
Commentary by Sri Ramanuja of Sri Sampradaya:

2.27 For what has originated, destruction is certain — it is seen to be inevitable. Similarly what has perished will inevitably originate. How should this be understood — that there is origination for that (entity)which has perished? It is seen that an existing entity only can originate and not a non-existent one. Origination, annihilation etc., are merely particular states of an existent entity. Now thread etc., do really exist. When arranged in a particular way, they are called clothes etc. It is seen that even those who uphold the doctrine that the effect is a new entity (Asatkarya-vadins) will admit this much that no new entity over and above the particular arrangement of threads is seen. It is not tenable to hold that this is the coming into being of a new entity, since, by the process of manufacture there is only attainment of a new name and special functions. No new entity emerges. Origination, annihilation etc., are thus particular stages of an existent entity. With regard to an entity which has entered into a stage known as origination, its entry into the opposite condition is called annihilation. Of an evolving entity, a seqence of evolutionary stages is inevitable. For instance, clay becomes a lump, jug, a potsherd, and (finally) powder. Here, what is called annihilation is the attainment of a succeeding stage by an entity which existed previously in a preceding stage. And this annihilation itself is called birth in that stage. Thus, the sequence called birth and annihilation being inevitable for an evolving entity, it is not worthy of you to grieve. Now Sri Krsna says that not even the slightest grief arising from seeing an entity passing from a previous existing stage to an opposite stage, is justifiable in regard to human beings etc.
Commentary by Sri Sridhara Swami of Rudra Sampradaya:

Why is there still no need to grieve? This is being given here in this verse. For one who is born death is certain when the results of the actions which originated it are exhausted. Similarly when one dies rebirth is indeed certain due to the reactions good or bad which one has to accept by the karmic actions accrured while one was in that physical body. Therefore for the inevitable fact of birth and death one should never subject oneself to lament for.
Commentary by Sri Madhvacharya of Brahma Sampradaya:

Why should one not grieve? Because a natural function is being enacted according to cosmic order.

Now begins the summation.

Therefore understanding the reality of the existence of birth and death there should be no delusion.
Commentary by Sri Keshava Kashmiri of Kumara Sampradaya:

How is this verse to be understood? For one who is born death is certain. This is because of the reality of a fixed time limit in regard to the continuation of actions relating to the coming into existence of one’s physical body as well as the longevity of one’s present physical body. The fact that this reality is inevitable is due to the nature of activities performed in one’s previous existence which causes one to be continuously connected to the cycle of birth and death in the present body. Therefore it does not behoove one to grieve over the unavoidable regarding the physical body which is dependant on actions.
Commentary by Sri Adi Shankaracharya of Advaita Sampradaya:

2.27 This being so, ‘death of anyone born’, etc. Hi, for; mrtyuh, death; jatasya, of anyone born; dhruvah, is certain; is without exception; ca, and mrtasya, of the dead; janmah, (re-) birth; is dhruvam, a certainly. Tasmat, therefore, this fact, viz birth and death, is inevitable. With regard to that (fact), apariharye, over an enevitable; arthe, fact; tvam, you; na arhasi, ought not; socitum, to grieve.
Commentary by Sri Abhinavagupta of Kaula Tantra Sampradaya:    

2.27 Jatasya etc. Destruction comes after birth, and after the destruction comes birth. Thus, this series of birth-and-death is like a circle. Hence to what extent is this to be lamented for ? Furthermore-
Sanskrit Shloka Without Transliteration Marks:

jatasya hi dhruvo mrtyur
dhruvam janma mrtasya ca
tasmad apariharye ‘rthe
na tvam socitum arhasi
Sanskrit to English Word for Word Meanings:

jātasya — of one who has taken his birth; hi — certainly; dhruvaḥ — a fact; mṛtyuḥ — death; dhruvam — it is also a fact; janma — birth; mṛtasya — of the dead; ca — also; tasmāt — therefore; aparihārye — of that which is unavoidable; arthe — in the matter; na — do not; tvam — you; śocitum — to lament; arhasi — deserve.