Mother's milk flows in Love of the babe
It is only such an emptied heart, yearning only for the exclusive love of Lord Guru Nanak which gets filled with Grace and Glory of Sri Guru Nanak Sahib.
Maan Da Dudh Bache De Piar Vich Utarda Hai
The milk of Grace overflows in torrents to the loving hearts, to the thirsting and yearning souls. The child-like innocence, pure and sincere love attracts this milk of grace the most.
The mother by nature feels the urgency of the child, no sooner than the babe wants it. As the milk of the mother flows spontaneously out of love of the babe, so does the milk of Grace and Love of the Lord flow uninterrupted in the yearning and pure devotion of a child of the Lord. The child does not have to demand for the milk; a mere impulse is sufficient to elicit a spontaneous response of the Lord.
The mother also feeds the child in sleep though the child is not aware of it. Similarly, Satguru, a true divine parent, spiritually feeds and nourishes the innocent, child-like devotees totally dependent on Satguru's grace.
Khali Ho Jao Je Guru Nanak Nu Milna Hai
Baba Narinder Singh Ji explained and elaborated further :
It is only in such a vacant heart that the Lord of Love steps in. Such a heart is totally empty of all desires. It does not even crave for Mukti. It does not seek moksha.