First of all, you have to consider how this planet is floating in the air. It is floating in the air, as it is stated in the Bhagavad-gītā, gām āviśya (BG 15.13). Because Kṛṣṇa enters within this . . . Kṛṣṇa, just like He enters within the atom. So He's the spirit, Supreme Spirit. So because the Supreme Spirit is within, therefore it is floating. Not that weightlessness. Weight is there, very heavy weight. Just like you can test: This body will float in the water so long you are living. And as soon as the spirit soul goes, then immediately the body goes down. The law of gravity acts, the so-called law of gravity. This is an example. A child, so long it is living you can take in one hand. But as soon as the child is dead, it becomes heavier. That is natural.
So everything is lighter as soon as it is spiritually advanced or there is spirit. Similarly, when we'll be spiritually advanced, then there will be no impediment. Now we cannot fly in the air, but when you are free from this body, your spirit soul, within a second you can go to the Vaikuṇṭhaloka. It is so light. Within a second. Tyaktvā dehaṁ punar janma naiti mām eti (BG 4.9).
So the conclusion is, when the world becomes overloaded with the demons, nondevotees, it becomes disturbed; it becomes . . . the mother earth feels load, very much load. So at that time, Kṛṣṇa comes. Therefore it is said, bhāra-avatāraṇāya: just to unload. And bhārāvatāraṇāyānye. Somebody says, bhuvo nāva ivodadhau. Just like if a ship or boat is overloaded, it is very dangerous. It can be drowned at any moment. So sīdantyā bhūri-bhāreṇa.
So mother earth was overloaded, and she approached Brahmā on account of overloading of these demons. She was feeling too much uncomfortable. And Brahmā is the chief living being within this universe. So when there is need, others approaches Brahmā, and Brahmā approaches Viṣṇu to reduce the overburden. Then He takes His incarnation, and that is stated in the Bhagavad-gītā, yadā yadā hi dharmasya glānir bhavati bhārata, abhyutthānam adharmasya (BG 4.7). Abhyutthānam adharmasya. "When the principles of religiosity becomes increased, at that time I appear," Kṛṣṇa says.
Just like take, for example, when there are too much criminals in the state, unlawful, lawlessness, the state becomes overburdened, disturbed, overburdened. At that time the administrators are puzzled what to do. Similarly, when the world is overburdened by atheists, demons, nonbelievers, the world becomes overburdened. Otherwise things will go on nicely. Just like in the state, if people abide by the laws nicely, then it is very easy to administer; things are going on very nicely. But if people become criminals, it becomes a overburden to the state administrators.