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.
(Bhagavad-Gita----2:27----purport).

One who has actually come to understand one's constitutional position as an eternal servitor of the Lord gives up all engagements save working in Kṛṣṇa consciousness. As already explained, buddhi-yoga means transcendental loving service to the Lord. Such devotional service is the right course of action for the living entity. Only misers desire to enjoy the fruit of their own work just to be further entangled in material bondage. Except for work in Kṛṣṇa consciousness, all activities are abominable because they continually bind the worker to the cycle of birth and death. One should therefore never desire to be the cause of work. Everything should be done in Kṛṣṇa consciousness, for the satisfaction of Kṛṣṇa. Misers do not know how to utilize the assets of riches which they acquire by good fortune or by hard labor. One should spend all energies working in Kṛṣṇa consciousness, and that will make one's life successful. Like misers, unfortunate persons do not employ their human energy in the service of the Lord.
(Bhagavad-Gita----2:49----purport).

When fire, light, day and the fortnight of the moon are mentioned, it is to be understood that over all of them there are various presiding deities who make arrangements for the passage of the soul. At the time of death, the mind carries one on the path to a new life. If one leaves the body at the time designated above, either accidentally or by arrangement, it is possible for him to attain the impersonal brahmajyoti. Mystics who are advanced in yoga practice can arrange the time and place to leave the body. Others have no control—if by accident they leave at an auspicious moment, then they will not return to the cycle of birth and death, but otherwise there is every possibility that they will have to return. However, for the pure devotee in Kṛṣṇa consciousness, there is no fear of returning, whether he leaves the body at an auspicious or inauspicious moment, by accident or arrangement.
(Bhagavad-Gita----8:24----purport).

One who is promoted to the higher planetary systems enjoys a longer duration of life and better facilities for sense enjoyment, yet one is not allowed to stay there forever. One is again sent back to this earth upon finishing the resultant fruits of pious activities. He who has not attained perfection of knowledge, as indicated in the Vedānta-sūtra (janmādy asya yataḥ), or, in other words, he who fails to understand Kṛṣṇa, the cause of all causes, becomes baffled about achieving the ultimate goal of life and is thus subjected to the routine of being promoted to the higher planets and then again coming down, as if situated on a Ferris wheel which sometimes goes up and sometimes comes down. The purport is that instead of being elevated to the spiritual world, from which there is no longer any possibility of coming down, one simply revolves in the cycle of birth and death on higher and lower planetary systems. One should better take to the spiritual world to enjoy an eternal life full of bliss and knowledge and never return to this miserable material existence.
(Bhagavad-Gita----9:21----purport).

The dirty things in the heart of a conditioned soul are the root cause of all troubles for him. A conditioned soul is surrounded by the manifold miseries of material existence, but on account of his gross ignorance he is unable to remove the troubles due to dirty things in the heart, accumulated during the long prison life in the material world. He is actually meant to serve the will of the Supreme Lord, but on account of the dirty things in the heart, he likes to serve his concocted desires. These desires, instead of giving him any peace of mind, create new problems and thus bind him to the cycle of repeated birth and death. These dirty things of fruitive work and empiric philosophy can be removed only by association with the Supreme Lord. The Lord, being omnipotent, can offer His association by His inconceivable potencies. Thus persons who are unable to pin their faith on the personal feature of the Absolute are given a chance to associate with His virāṭ-rūpa, or the cosmic impersonal feature of the Lord.
(Bhagavad-Gita----2:1:22----purport).

The time factor cannot affect the span of life of the devotees. In Bhagavad-gītā it is stated that a little execution of devotional service saves one from the greatest danger. The greatest danger is transmigration of the soul from one body to another, and only devotional service to the Lord can stop this process. It is stated in the Vedic literatures, hariṁ vinā na mṛtim taranti: without the mercy of the Lord, one cannot stop the cycle of birth and death. In Bhagavad-gītā it is stated that only by understanding the transcendental nature of the Lord and His activities, His appearance and disappearance, can one stop the cycle of death and go back to Him. The time factor is divided into many fractions of moments, hours, months, years, periods, seasons, etc.
(Bhagavad-Gita----3:21:18----purport).

It is indicated herein that the cycle of birth and death cannot be stopped unless one is a pure devotee of the Supreme Lord. It is said, hariṁ vinā na mṛtim taranti. One cannot surpass the cycle of birth and death unless one is favored by the Supreme Personality of Godhead. The same concept is confirmed herewith: one may take to the system of understanding the Absolute Truth by one's own imperfect sensory speculation, or one may try to realize the self by the mystic yoga process; but whatever one may do, unless he comes to the point of surrendering to the Supreme Personality of Godhead, no process can give him liberation. One may ask if this means that those who are undergoing so much penance and austerity by strictly following the rules and regulations are endeavoring in vain. The answer is given by Śrīmad-Bhāgavatam (10.2.32): ye 'nye 'ravindākṣa vimukta-māninaḥ.
(Bhagavad-Gita----3:25:41----purport).

The Bhāgavatam says that because nondevotees neglect the transcendental loving service of the lotus feet of the Personality of Godhead, their intelligence is not sufficient, and therefore these persons fall down. The living entity must have some activity. If he does not engage in the activity of transcendental service, he must fall down to material activity. As soon as one falls down to material activity, there is no rescue from the cycle of birth and death. It is stated here by Lord Kapila, "Without My mercy" (nānyatra mad bhagavataḥ). The Lord is stated here to be Bhagavān, the Supreme Personality of Godhead, indicating that He is full of all opulences and is therefore perfectly competent to deliver one from the cycle of birth and death. He is also called pradhāna because He is the Supreme. He is equal to everyone, but to one who surrenders to Him He is especially favorable. It is also confirmed in Bhagavad-gītā that the Lord is equal to everyone; no one is His enemy and no one is His friend. But to one who surrenders unto Him, He is especially inclined. By the grace of the Lord, simply by surrendering unto Him one can get out of this cycle of birth and death. Otherwise, one may go on in many, many lives and may many times attempt other processes for liberation.
(Bhagavad-Gita----3:25:41----purport).

Forgetfulness of one's relationship with Lord Viṣṇu, the Supreme Personality of Godhead, is the cause of one's repeated birth and death. A living entity is as eternal as the Supreme Lord, but due to his forgetfulness he is put into this material nature and transmigrates from one body to another, and when the body is destroyed, he thinks that he is also destroyed. Actually, this forgetfulness of his relationship with Lord Viṣṇu is the cause of his destruction. Anyone who revives his consciousness of the original relationship receives inspiration from the Lord. This does not mean that the Lord is someone's enemy and someone else's friend. He helps everyone; one who is not bewildered by the influence of material energy is saved, and one who is bewildered is destroyed. It is said, therefore, hariṁ vinā na mṛtim taranti: no one can be saved from the repetition of birth and death without the help of the Supreme Lord. It is therefore the duty of all living entities to take shelter of the lotus feet of Viṣṇu and thus save themselves from the cycle of birth and death.
(Bhagavad-Gita----3:29:31----purport).