Q: Why do we need the principle, “to be filled for the Creator?”
A: This principle allows our desires to stay alert and not wither. After a meal, we are normally satiated and our desire disappears. Just imagine what it would be like if we could have a great meal, and only increase our appetite. The more we eat the more our will to receive increases.
That is why we need the principle, “not for myself, but for the Creator.” Only in that situation does our vessel become unlimited, and during the spiritual progress we experience feelings that are indescribable to ordinary senses. Because the pleasure we have in this world is, as Kabbalists describe, a minute Light, a spark that comes down to this world; it is not even a dim Light, not to mention an infinite one.
The desire for advancement does not go away with the evolution of spirituality. On the contrary, your desires keep growing and so do your abilities. That is why you begin to receive satisfaction, meaning the Light, in an increasing degree.