avyaktādīni bhūtāni
vyakta-madhyāni bhārata
avyakta-nidhanāny eva
tatra kā paridevanā

Translation of Bhagavad Gita 2.28

All created beings are unmanifest in their beginning, manifest in their interim state, and unmanifest again when annihilated. So what need is there for lamentation?

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

Accepting that there are two classes of philosophers, one believing in the existence of the soul and the other not believing in the existence of the soul, there is no cause for lamentation in either case. Nonbelievers in the existence of the soul are called atheists by followers of Vedic wisdom. Yet even if, for argument’s sake, we accept this atheistic theory, there is still no cause for lamentation. Apart from the separate existence of the soul, the material elements remain unmanifested before creation. From this subtle state of nonmanifestation comes manifestation, just as from ether, air is generated; from air, fire is generated; from fire, water is generated; and from water, earth becomes manifested. From the earth, many varieties of manifestations take place. Take, for example, a big skyscraper manifested from the earth. When it is dismantled, the manifestation becomes again unmanifested and remains as atoms in the ultimate stage. The law of conservation of energy remains, but in course of time things are manifested and unmanifested—that is the difference. Then what cause is there for lamentation either in the stage of manifestation or in unmanifestation? Somehow or other, even in the unmanifested stage, things are not lost. Both at the beginning and at the end, all elements remain unmanifested, and only in the middle are they manifested, and this does not make any real material difference.

And if we accept the Vedic conclusion as stated in the Bhagavad-gita that these material bodies are perishable in due course of time (antavanta ime dehah) but that the soul is eternal (nityasyoktah saririnah), then we must remember always that the body is like a dress; therefore why lament the changing of a dress? The material body has no factual existence in relation to the eternal soul. It is something like a dream. In a dream we may think of flying in the sky, or sitting on a chariot as a king, but when we wake up we can see that we are neither in the sky nor seated on the chariot. The Vedic wisdom encourages self-realization on the basis of the nonexistence of the material body. Therefore, in either case, whether one believes in the existence of the soul or one does not believe in the existence of the soul, there is no cause for lamentation for loss of the body.Commentary by Sri Vishvanatha Chakravarthi Thakur of Gaudiya Sampradaya:

For the soul there is no birth and death (verse 20). For the body, birth and death are certain (verse 27). Thus the causes of lamentation have been eliminated. In this verse both causes are eliminated at once.

Devas, humans and animals are not visible before their birth. However, their subtle and gross bodies do exist invisibly in a potential form, from the existence of the causal ingredients such as earth. They become visible in the middle period, and invisible after death. Even at the time of mahapralaya these bodies exist in a subtle form through the continued existence in subtle form of karmas and other elements. Thus all living entities bodies are invisible before birth and after death, and are visible in the interval.

This is stated by the personified Vedas in the Bhagavatam:

sthira-cara-jatayah syur ajayottha-nimitta- vujah

O eternally liberated, transcendental Lord, Your material energy causes the various moving and nonmoving species of life to appear by activating their material desires, but only when and if You sport with her by briefly glancing at her. SB 10.87.29.

What cause is there for lamentation (paridevana)?

As Narada says:

yan manyase dhruvam lokam adhruvam va na cobhayam sarvatha na hi socyas te snehad anyatra mohajat

O King, in all circumstances, whether you consider the soul to be an eternal principle, or the material body to be perishable, or everything to exist in the impersonal Absolute Truth, or everything to be an inexplicable combination of matter and spirit, feelings of separation are due only to illusory affection and nothing more SB 1.13.44Commentary by Sri Ramanuja of Sri Sampradaya:

2.28 Human beings etc., (i.e., bodies) exist as entities; their previous stages are unknown, their middle stages in the form of man etc., are known, and their (final) and future stages are unknown. As they thus exist in their own natural stages, there is no cause for grief. After thus saying that there is no cause for grief even according to the view which identifies the body with the self, Sri Krsna proceeds to say that it is hard to find one who can be said to have truly perceived the Atman or spoken about It or heard about It or gained a true conception of It by hearing. For the Atman, which is actually different from the body, is of a wonderful nature.
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:

2.28 It is not reasonable to grieve even for beings which are constituted by bodies and organs, since ‘all beings remain unmanifest’ etc. (Bharata, O descendant of Bharata;) bhutani, all beings, avyaktaduni, remain unmainfest in the beginning. Those beings, viz sons, friends, and others, constituted by bodies and organs, [Another reading is karya-karana-sanghata, aggregates formed by material elements acting as causes and effects.-Tr.] who before their origination have unmanifestedness (avyakta), invisibility, nonperception, as their beginning (adi) are avyaktaadini. Ca, and; after origination, before death, they become vyakta-madhyani, manifest in the middle. Again, they eva, certainly; become avyakta-nidhanani, unmanifest after death. Those which have unmanifestness (avyakta), invisibility, as their death (nidhana) are avyakta-nidhanani. The idea is that even after death they verily attain unmanifestedness. Accordingly has it been said: ‘They emerged from invisibility, and have gone back to invisibility. They are not yours, nor are you theirs. What is this fruitless lamentation!’ (Mbh. St. 2.13). Ka, what; paridevana, lamentation, or what prattle, can there be; tatra, with regard to them, i.e. with regard to beings which are objects of delusion, which are invisible, (become) visible, (and then) get destroyed!
Commentary by Sri Abhinavagupta of Kaula Tantra Sampradaya:    

2.28 Avyaktadini etc. Whether beings are permanent or impermanent, this much is certain : The person, who laments over a given object – as far as that person is concerned, that object is at the beginning unmanifest and at the end also it is unmanifest. Its manifestation in between is therefore a deviation from its natural state, Rather, there may be need to lament over the deviation from natural state and nor over the natural state [itself]. Further, whatever has been approved as its root cause, that itself permanently exhibits, within itself, a variety of different and endless creation, sustenance and absorption as its own manifold nature, in a set pattern. Hence what is the necessity for lamenting over the same nature of this (its effect) ? And enowed with the above mentioned nature-
Sanskrit Shloka Without Transliteration Marks:

avyaktadini bhutani
vyakta-madhyani bharata
avyakta-nidhanany eva
tatra ka paridevana
Sanskrit to English Word for Word Meanings:

avyakta-ādīni — in the beginning unmanifested; bhūtāni — all that are created; vyakta — manifested; madhyāni — in the middle; bhārata — O descendant of Bharata; avyakta — nonmanifested; nidhanāni — when vanquished; eva — it is all like that; tatra — therefore; kā — what; paridevanā — lamentation.