PURPORT

A tree has no consciousness: when cut, it feels no pain. But Narada Muni wanted the consciousness of Nalakuvara and Manigriva to continue, so that even after being released from the life of trees, they would not forget the circumstances under which they had been punished. Therefore, to bestow upon them special favor, Narada Muni arranged things in such a way that after being released, they would be able to see Krsna in Vrndavana and thus revive their dormant bhakti.

Each day of the demigods in the upper planetary system equals six months of our measurement. Although the demigods in the upper planetary system are attached to material enjoyment, they are all devotees, and therefore they are called demigods. There are two kinds of persons, namely the devas and the asuras. Asuras forget their relationship with Krsna (asuram bhavam asritah), whereas the devas do not forget.

dvau bhuta-sargau loke 'smin
daiva asura eva ca
visnu-bhaktah smrto daiva
asuras tad-viparyayah
(Padma Purana)

The distinction between a pure devotee and a karma-misra devotee is this: a pure devotee does not desire anything for material enjoyment, whereas a mixed devotee becomes a devotee to become a first-class enjoyer of this material world. One who is in direct touch with the Supreme Personality of Godhead in devotional service remains pure, uncontaminated by material desires (anyabhilasita-sunyam jnana-karmady-anavrtam).

By karma-misra-bhakti one is elevated to the celestial kingdom, by jnana-misra-bhakti one is able to merge in the Brahman effulgence, and by yoga-misra-bhakti one is able to realize the omnipotency of the Supreme Personality of Godhead. But pure bhakti does not depend on karma, jnana or yoga, for it simply consists of loving affairs. The liberation of the bhakta, therefore, which is called not just mukti but vimukti, surpasses the five other kinds of liberation-sayujya, sarupya, salokya, sarsti and samipya. A pure devotee always engages in pure service (anukulyena krsnanusilanam bhaktir uttama). Taking birth in the upper planetary system as a demigod is a chance to become a further purified devotee and go back home, back to Godhead. Narada Muni indirectly gave Manigriva and Nalakuvara the greatest opportunity by his so-called curse.