SB 11.22.39

viṣayābhiniveśena
 nātmānaṁ yat smaret punaḥ
jantor vai kasyacid dhetor
 mṛtyur atyanta-vismṛtiḥ
Synonyms: 
viṣaya — in (new) objects of perception; abhiniveśena — because of absorption; na — not; ātmānam — his previous self; yat — the situation in which; smaret — remembers; punaḥ — any more; jantoḥ — of the living entity; vai — indeed; kasyacit hetoḥ — for any reason or other; mṛtyuḥ — known as death; atyanta — total; vismṛtiḥ — forgetfulness.
Translation: 
When the living entity passes from the present body to the next body, which is created by his own karma, he becomes absorbed in the pleasurable and painful sensations of the new body and completely forgets the experience of the previous body. This total forgetfulness of one’s previous material identity, which comes about for one reason or another, is called death.
Purport: 

Depending on one’s karma, or fruitive activities, one may achieve a beautiful, wealthy or powerful body or be degraded to an abominable condition of life. Taking birth in heaven or in hell, the living entity learns to completely identify his ego with the new body and thus becomes absorbed in the pleasure, fear, opulence or suffering of the new body, completely forgetting the experiences of the previous body. Death occurs when the specific karma allotted to a physical body is finished. Since that particular body’s karma is used up, it can no longer act upon one’s mind; in that way one forgets the previous body. The new body is created by nature so that one can experience the karma currently in effect. Consequently one’s entire consciousness becomes absorbed in one’s current body in order that one can fully experience the results of his previous activities. Because the living entity falsely identifies himself as the body, bodily death is experienced as death of the soul. Actually, however, the soul is eternal and is never subject to creation or annihilation. This analytic knowledge of self-realization is easily understood in Kṛṣṇa consciousness.