During the dry season, very little water comes from the river into the ocean. Does the ocean dry up? During the monsoons, sometimes the river becomes several miles wide and billions of liters of water pour in every minute in the ocean. Does the ocean overflow? Ocean is ever being filled but is always still. Why? Because the ocean is pure and so deep in itself that it is not disturbed by the external forces coming in.
Compare the ocean with a puddle of water. During the dry season, it completely dries up; we cannot get even a drop out of it. And during the rainy season, it floods over. Why? Because there is no depth.
If you are full within, if you are deep, none of the external forces can disturb you. But if you are empty, you are completely affected by everything that comes in through your senses. (Based on Bhagavad-gita 2.70)