The eternal living entity transmigrates from one material body to another by the law of karma, and material bodies are perishable by their fundamental structures. Therefore there is nothing to be lamented in the case of the soul's being transferred into another body, or the material body's perishing at a certain stage. There are others also who believe in the merging of the spirit soul in the Supreme Spirit when it is uncovered by the material encagement, and there are others also who do not believe in the existence of spirit or soul, but believe in tangible matter. In our daily experience we find so many transformations of matter from one form to another, but we do not lament such changing features. In either of the above cases, the force of divine energy is uncheckable; no one has any hand in it, and thus there is no cause of grief.
(Srimad Bhagavatam---1:13:44---purport).

The entire universe is full of miseries, and therefore the inhabitants of this material universe are always shedding tears out of intense grief. There is a great ocean of water made from such tears, but for one who surrenders unto the Supreme Personality of Godhead, the ocean of tears is at once dried up. One need only see the charming smile of the Supreme Lord. In other words, the bereavement of material existence immediately subsides when one sees the charming smile of the Lord.
(Srimad Bhagavatam---3:28:32---purport).

For formality's sake, when a man is lying on his deathbed, his relatives come to him, and sometimes they cry very loudly, addressing the dying man: "Oh, my father!" "Oh, my friend!" or "Oh, my husband!" In that pitiable condition the dying man wants to speak with them and instruct them of his desires, but because he is fully under the control of the time factor, death, he cannot express himself, and that causes him inconceivable pain. He is already in a painful condition because of disease, and his glands and throat are choked up with mucus. He is already in a very difficult position, and when he is addressed by his relatives in that way, his grief increases.
(Srimad Bhagavatam---3:30:17---purport).

Even though one is very much advanced in devotional service, if he commits offenses at the feet of a Vaiṣṇava, his advancement is all spoiled. In the śāstras it is found that a very great yogī, Durvāsā Muni, committed a vaiṣṇava-aparādha and thus for one full year had to travel all over the universe, even to Vaikuṇṭhaloka, to defend himself from the offense. At last, even when he approached the Supreme Personality of Godhead in Vaikuṇṭha, he was refused protection. Therefore one should be very careful about committing offenses at the feet of a Vaiṣṇava. The most grievous type of vaiṣṇava-aparādha is called gurv-aparādha, which refers to offenses at the lotus feet of the spiritual master. In the chanting of the holy name of the Supreme Personality of Godhead, this gurv-aparādha is considered the most grievous offense. Guror avajñā śruti-śāstra-nindanam (Padma Purāṇa). Among the ten offenses committed against the chanting of the holy name, the first offenses are disobedience of the spiritual master and blasphemy of the Vedic literature.
(Srimad Bhagavatam---4:21:37---purport).

A devotee who is not dependent on the ordinary course of activities, who is pure, expert, without cares, free from all pains, and who does not strive for some result, is very dear to Me. One who grasps neither pleasure or grief who neither laments nor desires, and who renounces both auspicious and inauspicious things, is very dear to Me.
(Teachings of Lord Caitanya).