mam upetya punar janma
samsiddhim paramam gatah
After attaining Me, the great souls, who are yogis in devotion, never return to this temporary world, which is full of miseries, because they have attained the highest perfection.
Commentary by Srila Prabhupada
Since this temporary material world is full of the miseries of birth, old age, disease and death, naturally he who achieves the highest perfection and attains the supreme planet, Krishnaloka, Goloka Vrindavana, does not wish to return. The supreme planet is described in Vedic literature as avyakta and akshara and parama gati; in other words, that planet is beyond our material vision, and it is inexplicable, but it is the highest goal, the destination for the mahatmas (great souls). The mahatmas receive transcendental messages from the realized devotees and thus gradually develop devotional service in Krishna consciousness and become so absorbed in transcendental service that they no longer desire elevation to any of the material planets, nor do they even want to be transferred to any spiritual planet. They only want Krishna and Krishna’s association, and nothing else. That is the highest perfection of life. This verse specifically mentions the personalist devotees of the Supreme Lord, Krishna. These devotees in Krishna consciousness achieve the highest perfection of life. In other words, they are the supreme souls.
Commentary by Sri Visvanatha Cakravarti Thakur
What happens to the person who attains you? Attaining me, they do not attain birth again, which is full of suffering (duhkhalayam) and temporary, but attain a birth similar to mine, which is full of joy, because it is eternal. According to the Amara Kosa, sasvata means certain, eternal, continual, everlasting. When 1 take my birth, which is full of joy, spiritual, and eternal, in the house of Vasudeva, my devotees, being my eternal associates, will also take such birth, and not at other times. Other devotees attain perfection, but those who exclusively think of me (ananya cetah), attain the very highest perfection (paramam samsiddhim) – a position as an assistant in my pastimes. This indicates the observable superiority of kevala bhakta above all other types of devotees mentioned.