Certainly Mother Yaśodā had executed many, many pious activities, as a result of which she got the Absolute Truth, the Supreme Personality of Godhead, as her son who sucked milk from her breast. Similarly, Nanda Mahārāja also must have performed many great sacrifices and pious activities for Lord Kṛṣṇa to become his son and address him as "Father." But it is surprising that Vasudeva and Devakī did not enjoy the transcendental bliss of Kṛṣṇa's childhood pastimes, although Kṛṣṇa was their real son. The childhood pastimes of Kṛṣṇa are glorified even today by many sages and saintly persons, but Vasudeva and Devakī could not enjoy such childhood pastimes personally. The reason for this was explained by Śukadeva Gosvāmī to Mahārāja Parīkṣit as follows.
When the best of the Vasus, named Droṇa, and his wife Dharā were ordered to increase progeny by Lord Brahmā, they said unto him, "Dear Father, we are seeking your benediction. When we take birth again within the universe, may the Supreme Lord Kṛṣṇa in His most attractive feature of childhood absorb our whole attention. May our dealings with Him be so powerful that simply by hearing of these childhood activities of His, anyone will very easily cross over the nescience of birth and death." Lord Brahmā agreed to give them the benediction, and as a result the same Droṇa appeared as Nanda Mahārāja in Vṛndāvana, and the same Dharā appeared as Mother Yaśodā, the wife of Nanda Mahārāja.
In this way, Nanda Mahārāja and his wife, Mother Yaśodā, developed their unalloyed devotion for the Supreme Personality of Godhead, having gotten Him as their son. And all the gopīs and cowherd men who were associates of Kṛṣṇa naturally developed their own different feelings of love for Kṛṣṇa.
Therefore, just to fulfill the benediction of Lord Brahmā, Lord Kṛṣṇa appeared along with His plenary expansion, Balarāma, and performed all kinds of childhood pastimes in order to increase the transcendental pleasure of all the residents of Vṛndāvana.