Samyaktvam cha (21)
In spite of all their - from the human point of view - extraordinary abilities, the inhabitants of 'heavenly' regions (devas) are not free from karma. Any negative karma that might oppose the life-circumstances of devas has only receded into an inactive, latent state and will reactivate after the karmas (the emotional tendencies) that caused the celestial birth(s) are dissolved.
According to the Tattvarthasutra an embodiment in 'heavenly' regions is not the first step towards enlightenment, but only indicates the deep desire for a certain range of positive experiences. Any deva, who - after the dissolution of this (positive) karma - wants to dissolve all his karma needs to incarnate in 'our' world, since this is the only place within the context of this universe, where positive and negative karma can manifest simultaneously.
Therefore any deva who wants to reach freedom from all karmic limitations needs to bind karma that will enable his incarnation in our world - i.e. he needs to desire a human form. He generates this special karma by orienting towards growth. Once he follows this red line that leads from his present position to ultimate freedom, he will automatically acquire a human body that provides access to our world.
Yet only devas need to bind this type of karma; in our world the orientation towards growth causes no new karmic bonds.