All the worlds, O Arjuna, including the realm of Brahma, are subject to return. But after attaining Me, O son of Kunti, there is no rebirth. (Bhagavad Gita Chapter 8, verse 16).
Everything is subject to time, the all-destroyer. And time means change, coming and going, birth and death, creation and destruction. Therefore, where time rules there is no rest. All the spheres, all the worlds, or lokas, are subject to time. Heaven will pass away and earth will pass away. But One will remain eternally. That is the Deity, the Spirit. Time cannot attack Him. He snatches high and low, strong and weak, young and old, but (though he has a thousand arms) time cannot reach up to the Spirit. God is beyond time, space and causation. He is never subject to change, and in Him alone we find rest. In Him alone is our true home, our resting place, the end of our journey. In other spheres we live and enjoy for thousands of years, but it comes to an end. We reach those worlds or heavens through good karma, by worshipping some deity. All actions proceed from desire that brings us the enjoyment of higher spheres, that makes us perform such acts as will take us there. And as those good deeds are numbered, so also the result of those deeds, our abode in some heaven, must come to an end.
But when one has attained Him, says Bhagavan, there is no rebirth. Those who attain the Spirit are desireless. Otherwise, they could not have gone to Him. Only by leaving behind desires and attachments, can we go to God. We must desire Him and Him alone. It is only then that we come to rest and it is only then that we can taste immortality. Only in Him is salvation. Only the man of renunciation gets freedom.