Fruitive work which has perpetually engaged the living being is compared to the banyan tree in the Bhagavad-gītā, for it is certainly very deeply rooted. As long as the propensity for enjoying the fruit of work is there, one has to continue the transmigration of the soul from one body or place to another, according to one's nature of work. The propensity for enjoyment may be turned into the desire for serving the mission of the Lord, By doing so, one's activity is changed into karma-yoga, or the way by which one can attain spiritual perfection while engaging in the work for which he has a natural tendency. Here the word ātmā indicates the categories of all fruitive work. The conclusion is that when the result of all fruitive and other work is dovetailed with the service of the Lord, it will cease to generate further karma and will gradually develop into transcendental devotional service, which will not only cut off completely the root of the banyan tree of work but will also carry the performer to the lotus feet of the Lord.
(Srimad Bhagavatam--1:5:34---purport).

Constant transmigration of the soul from one body to another is the cause of suffering in material existence. This conditional life in material existence is called saṁsāra. One may perform good work and take his birth in a very nice material condition, but the process under which birth and death take place is like a terrible fire. Śrī Viśvanātha Cakravartī Ṭhākura, in his prayer to the spiritual master, has described this. Saṁsāra, or the repetition of birth and death, is compared to a forest fire. A forest fire takes place automatically, without anyone's endeavor, by the friction of dried wood, and no fire department or sympathetic person can extinguish it. The raging forest fire can be extinguished only when there is a constant downpour of water from a cloud. The cloud is compared to the mercy of the spiritual master. By the grace of the spiritual master the cloud of the mercy of the Personality of Godhead is brought in, and then only, when the rains of Kṛṣṇa consciousness fall, can the fire of material existence be extinguished. This is also explained here. In order to find freedom from the stereotyped conditional life of material existence, one has to take shelter of the lotus feet of the Lord, not in the manner in which the impersonalists indulge, but in devotional service, chanting and hearing of the activities of the Lord. Only then can one be freed from the actions and reactions of material existence.
(Srimad Bhagavatam---3:21:17---purport).

Actually the living entity is in the subtle body, even though the gross body apparently changes and even though he inhabits the gross body on various planets. All the activities performed by the living entity in the subtle body are called illusory because they are not permanent. Liberation means getting out of the clutches of the subtle body. Liberation from the gross body simply involves the transmigration of the soul from one gross body to another. When the mind is educated in Kṛṣṇa consciousness, or higher consciousness in the mode of goodness, one is transferred either to the upper, heavenly planets or to the spiritual world, the Vaikuṇṭha planets. One therefore has to change his consciousness by cultivating knowledge received from Vedic instructions from the Supreme Personality of Godhead through the disciplic succession.
(Srimad Bhagavatam---4:29:61---purport).

One's consciousness, therefore, must always be absorbed in Kṛṣṇa, and then one's life will be successful. Otherwise the mind will carry the soul to another material body. The soul will be placed in the semen of a father and discharged into the womb of a mother. The semen and ovum create a particular type of body according to the form of the father and mother, and when the body is mature, the soul emerges in that body and begins a new life. This is the process of transmigration of the soul from one body to another (tathā dehāntara-prāptiḥ (B.G-2:13). Unfortunately, those who are less intelligent think that when the body disappears, everything is finished. The entire world is being misled by such fools and rascals. But as stated in Bhagavad-gītā (2.20), na hanyate hanyamāne śarīre. The soul does not die when the body is destroyed. Rather, the soul takes on another body.
(Srimad Bhagavatam---10:1:41---purport).

"As the embodied soul continuously passes, in this body, from boyhood to youth to old age, the soul similarly passes into another body at death. The self-realized soul is not bewildered by such a change." Such verses from the Bhagavad-gītā or any other Vedic literature give valuable instructions on the occasion of someone's passing away. By discussing such instructions from the Bhagavad-gītā or Śrīmad-Bhāgavatam, a sober man can certainly understand that the soul never dies but rather passes from one body to another. This is called transmigration of the soul. A soul comes into this material world and creates bodily relationships with a father, a mother, sisters, brothers, a wife and children, but all these relationships pertain to the body, not the soul. Therefore, as described in the Bhagavad-gītā, dhīras tatra na muhyati: one who is sober is not disturbed by such phenomenal changes within this material world. Such instructions are called tattva-kathā, or real truth.
(Sri Caitanya Caritamrta---1:16:23---purport).

Due to our association with the modes of material nature, we get different types of bodies, good and bad. One cannot be liberated from the cycle of birth and death, known as transmigration of the soul, unless one is completely freed from all sinful activities. The best process, therefore, is to take to Kṛṣṇa consciousness. One cannot take to Kṛṣṇa consciousness without being freed from all sinful activities. Naturally one who is very serious about Kṛṣṇa consciousness is freed from all sinful activity. Consequently a devotee is never inclined to commit sins. If one is pressured by the law or obligations to give up sinful activity, one cannot do so. However, if one takes to Kṛṣṇa consciousness, he can very easily give up all sinful activity.
(Sri Caitanya Caritamrta---2:22:142---purport).