Why Arjuna was puzzled by Kṛṣṇa’s going to see Kāraṇārṇavaśāyī Viṣṇu in the spiritual world is fully discussed in the commentaries of Śrīla Viśvanātha Cakravartī Ṭhākura, as follows. It is understood from the speech of Mahā-Viṣṇu that He was very eager to see Kṛṣṇa. It may be said, however, that since Mahā-Viṣṇu took away the brāhmaṇa’s sons, He must certainly have gone to Dvārakā to do so. Therefore, why did He not see Kṛṣṇa there? A possible answer is that unless Kṛṣṇa gives His permission, He cannot be seen even by Mahā-Viṣṇu, lying in the Causal Ocean of the spiritual world. Thus Mahā-Viṣṇu took away the brāhmaṇa’s sons one after another just after their births so that Kṛṣṇa would come personally to the Casual Ocean to retrieve them, and then Mahā-Viṣṇu would be able to see Him there. If that is so, the next question is this: Why would Mahā-Viṣṇu come to Dvārakā personally if He were not able to see Kṛṣṇa? Why did He not send some of His associates to take away the sons of the brāhmaṇa? A possible answer is that it is very difficult to put any of the citizens of Dvārakā into trouble in the presence of Kṛṣṇa. Therefore, because it was not possible for any of Mahā-Viṣṇu’s associates to take away the brāhmaṇa’s sons, He personally came to take them.
Another question may also be raised: The Lord is known as brahmaṇya-deva, the worshipable Deity of the brāhmaṇas, so why was He inclined to put a brāhmaṇa into such a terrible condition of lamentation over one son after another until the tenth son was taken away? The answer is that Lord Mahā-Viṣṇu was so eager to see Kṛṣṇa that He did not hesitate even to give trouble to a brāhmaṇa. Although giving trouble to a brāhmaṇa is a forbidden act, Lord Viṣṇu was prepared to do anything in order to see Kṛṣṇa—He was so eager to see Him. After losing each of his sons, the brāhmaṇa would come to the gate of the palace and accuse the King of not being able to give the brāhmaṇas protection and of thus being unfit to sit on the royal throne. It was Mahā-Viṣṇu’s plan that the brāhmaṇa would accuse the kṣatriyas and Kṛṣṇa, and Kṛṣṇa would be obliged to come see Him to take back the brāhmaṇa’s sons.
Still another question may be raised: If Mahā-Viṣṇu cannot see Kṛṣṇa, then how was Kṛṣṇa obliged to come before Him after all to take back the sons of the brāhmaṇa? The answer is that Lord Kṛṣṇa went to see Lord Mahā-Viṣṇu not exactly to take away the sons of the brāhmaṇa but only for Arjuna’s sake. His friendship with Arjuna was so intimate that when Arjuna prepared himself to die by entering a fire, Kṛṣṇa wanted to give him complete protection. Arjuna, however, would not desist from entering the fire unless the sons of the brāhmaṇa were brought back. Therefore Kṛṣṇa promised him, “I shall bring back the brāhmaṇa’s sons. Do not try to commit suicide.”
If Lord Kṛṣṇa were going to see Lord Viṣṇu only to reclaim the sons of the brāhmaṇa, then He would not have waited until the tenth son was taken. But when the tenth son was taken away by Lord Mahā-Viṣṇu, and Arjuna was therefore ready to enter the fire because his promise was going to prove false, that serious situation made Lord Kṛṣṇa decide to go with Arjuna to see Mahā-Viṣṇu. It is said that Arjuna is an empowered incarnation of Nara-Nārāyaṇa. He is even sometimes called Nara-Nārāyaṇa. The Nara-Nārāyaṇa incarnation is also one of Lord Viṣṇu’s plenary expansions. Therefore, when Kṛṣṇa and Arjuna went to see Lord Viṣṇu, it is to be understood that Arjuna visited in His Nara-Nārāyaṇa capacity, just as Kṛṣṇa, when He displayed His pastimes in Dvārakā, acted in His Vāsudeva capacity.
After visiting the spiritual world, Arjuna concluded that whatever opulence anyone can show within the material or spiritual worlds is all a gift of Lord Kṛṣṇa. Lord Kṛṣṇa is manifested in various forms, as viṣṇu-tattva and jīva-tattva, or, in other words, as svāṁśa and vibhinnāṁśa. Viṣṇu-tattva is known as svāṁśa, and jīva-tattva is known as vibhinnāṁśa. He can, therefore, display Himself by His different transcendental pastimes, in the portion of either svāṁśa or vibhinnāṁśa, as He likes, but still He remains the original Supreme Personality of Godhead.