Although Kṛṣṇa is manifested before Arjuna in His universal form, Arjuna remembers his friendly relationship with Kṛṣṇa and is therefore asking pardon and requesting Kṛṣṇa to excuse him for the many informal gestures which arise out of friendship. He is admitting that formerly he did not know that Kṛṣṇa could assume such a universal form, although Kṛṣṇa explained it as his intimate friend. Arjuna did not know how many times he may have dishonored Kṛṣṇa by addressing Him "O my friend," "O Kṛṣṇa," "O Yādava," etc., without acknowledging His opulence.
(Bhagavad-Gita----11:41-42----purport).

But Kṛṣṇa is so kind and merciful that in spite of such opulence He played with Arjuna as a friend. Such is the transcendental loving reciprocation between the devotee and the Lord. The relationship between the living entity and Kṛṣṇa is fixed eternally; it cannot be forgotten, as we can see from the behavior of Arjuna. Although Arjuna has seen the opulence in the universal form, he cannot forget his friendly relationship with Kṛṣṇa.
(Bhagavad-Gita----11:41-42----purport).

The Supreme Personality of Godhead, Kṛṣṇa, is worshipable as a father is worshipable for his son. He is the spiritual master because He originally gave the Vedic instructions to Brahmā and presently He is also instructing Bhagavad-gītā to Arjuna; therefore He is the original spiritual master, and any bona fide spiritual master at the present moment must be a descendant in the line of disciplic succession stemming from Kṛṣṇa. Without being a representative of Kṛṣṇa, one cannot become a teacher or spiritual master of transcendental subject matter.
(Bhagavad-Gita----11:43----purport).

Kṛṣṇa's devotees relate to Kṛṣṇa in various relationships; one might treat Kṛṣṇa as a son, or one might treat Kṛṣṇa as a husband, as a friend, or as a master. Kṛṣṇa andArjuna are related in friendship. As the father tolerates, or the husband or a master tolerates, so Kṛṣṇa tolerates.
(Bhagavad-Gita----11:44----purport).

Arjuna is always in confidence with Kṛṣṇa because he is a very dear friend, and as a dear friend is gladdened by his friend's opulence, Arjuna is very joyful to see that his friend Kṛṣṇa is the Supreme Personality of Godhead and can show such a wonderful universal form. But at the same time, after seeing that universal form, he is afraid that he has committed so many offenses to Kṛṣṇa out of his unalloyed friendship. Thus his mind is disturbed out of fear, although he had no reason to fear. Arjuna therefore is asking Kṛṣṇa to show His Nārāyaṇa form, because He can assume any form. This universal form is material and temporary, as the material world is temporary.
(Bhagavad-Gita----11:45----purport).

Arjuna wanted to see the universal form of the Supreme Lord, so Lord Kṛṣṇa, out of His mercy upon His devotee Arjuna, showed His universal form, full of effulgence and opulence. This form was glaring like the sun, and its many faces were rapidly changing. Kṛṣṇa showed this form just to satisfy the desire of His friend Arjuna. This form was manifested by Kṛṣṇa through His internal potency, which is inconceivable by human speculation. No one had seen this universal form of the Lord before Arjuna, but because the form was shown toArjuna, other devotees in the heavenly planets and in other planets in outer space could also see it. They had not seen it before, but because of Arjuna they were also able to see it.
(Bhagavad-Gita----11:47----purport).

Kṛṣṇa showed His universal form to Arjuna just to show him that these people were already killed for their unlawful action. That scene was shown to Arjuna because devotees are always peaceful and they cannot perform such horrible actions. The purpose of the revelation of the universal form was shown; now Arjuna wanted to see the four-armed form, and Kṛṣṇa showed him. A devotee is not much interested in the universal form, for it does not enable one to reciprocate loving feelings. Either a devotee wants to offer his respectful worshipful feelings, or he wants to see the two-handed Kṛṣṇa form so that he can reciprocate in loving service with the Supreme Personality of Godhead.
(Bhagavad-Gita----11:49----purport).

When Kṛṣṇa appeared as the son of Vasudeva and Devakī, He first of all appeared as four-armed Nārāyaṇa, but when He was requested by His parents, He transformed Himself into an ordinary child in appearance. Similarly, Kṛṣṇa knew that Arjuna was not interested in seeing a four-handed form, but since Arjuna asked to see this four-handed form, Kṛṣṇa also showed him this form again and then showed Himself in His two-handed form. The word saumya-vapuḥ is very significant. Saumya-vapuḥ is a very beautiful form; it is known as the most beautiful form. When He was present, everyone was attracted simply by Kṛṣṇa's form, and because Kṛṣṇa is the director of the universe, He just banished the fear ofArjuna, His devotee, and showed him again His beautiful form of Kṛṣṇa. In the Brahma-saṁhitā (5.38) it is stated, premāñjana-cchurita-bhakti-vilocanena: only a person whose eyes are smeared with the ointment of love can see the beautiful form of Śrī Kṛṣṇa.
(Bhagavad-Gita----11:50----purport).

"There is nothing superior to My form as the Personality of Godhead Kṛṣṇa." He also says elsewhere in Bhagavad-gītā, aham ādir hi devānām: "I am the origin of all the demigods." And after understanding Bhagavad-gītā from Kṛṣṇa, Arjuna also confirms this in the following words: paraṁ brahma paraṁ dhāma pavitram-paramaṁ bhavān, "I now fully understand that You are the Supreme Personality of Godhead, the Absolute Truth, and that You are the refuge of everything." Therefore the universal form which Kṛṣṇa showed to Arjuna is not the original form of God. The original is the Kṛṣṇa form. The universal form, with its thousands and thousands of heads and hands, is manifest just to draw the attention of those who have no love for God. It is not God's original form.
(Bhagavad-Gita----11:54----purport).

Those who worship the Supreme Lord directly by devotional service are called personalists. Those who engage themselves in meditation on the impersonal Brahman are called impersonalists. Arjuna is here questioning which position is better. There are different ways to realize the Absolute Truth, but Kṛṣṇa indicates in this chapter that bhakti-yoga, or devotional service to Him, is the highest of all. It is the most direct, and it is the easiest means for association with the Godhead.
(Bhagavad-Gita----12:1----purport).

And so, factually, there are two classes of transcendentalists. Now Arjuna is trying to settle the question of which process is easier and which of the classes is most perfect. In other words, he is clarifying his own position because he is attached to the personal form of Kṛṣṇa. He is not attached to the impersonal Brahman. He wants to know whether his position is secure. The impersonal manifestation, either in this material world or in the spiritual world of the Supreme Lord, is a problem for meditation. Actually, one cannot perfectly conceive of the impersonal feature of the Absolute Truth. Therefore Arjuna wants to say, "What is the use of such a waste of time?" Arjuna experienced in the Eleventh Chapter that to be attached to the personal form of Kṛṣṇa is best because he could thus understand all other forms at the same time and there was no disturbance to his love for Kṛṣṇa. This important question asked of Kṛṣṇa by Arjuna will clarify the distinction between the impersonal and personal conceptions of the Absolute Truth.
(Bhagavad-Gita----12:1----purport).

In answer to Arjuna's question, Kṛṣṇa clearly says that he who concentrates upon His personal form and who worships Him with faith and devotion is to be considered most perfect in yoga. For one in such Kṛṣṇa consciousness there are no material activities, because everything is done for Kṛṣṇa. A pure devotee is constantly engaged. Sometimes he chants, sometimes he hears or reads books about Kṛṣṇa, or sometimes he cooks prasādam or goes to the marketplace to purchase something for Kṛṣṇa, or sometimes he washes the temple or the dishes—whatever he does, he does not let a single moment pass without devoting his activities to Kṛṣṇa. Such action is in full samādhi.
(Bhagavad-Gita----12:2----purport).

The devotee does not desire any achievement other than pleasing the Supreme Personality of Godhead. His life's mission is to please Kṛṣṇa, and he can sacrifice everything for Kṛṣṇa's satisfaction, just as Arjuna did in the Battle of Kurukṣetra. The process is very simple: one can devote himself in his occupation and engage at the same time in chanting Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare. Such transcendental chanting attracts the devotee to the Personality of Godhead.
(Bhagavad-Gita----12:6-7----purport).

Arjuna was inquisitive about prakṛti (nature), puruṣa (the enjoyer), kṣetra (the field), kṣetra-jña (its knower), and knowledge and the object of knowledge. When he inquired about all these, Kṛṣṇa said that this body is called the field and that one who knows this body is called the knower of the field. This body is the field of activity for the conditioned soul. The conditioned soul is entrapped in material existence, and he attempts to lord it over material nature.
(Bhagavad-Gita----13:1-2----purport).

Unless one is situated in Kṛṣṇa consciousness, his material consciousness will oblige him to transfer from one body to another because he has material desires since time immemorial. But he has to change that conception. That change can be effected only by hearing from authoritative sources. The best example is here: Arjuna is hearing the science of God from Kṛṣṇa. The living entity, if he submits to this hearing process, will lose his long-cherished desire to dominate material nature, and gradually and proportionately, as he reduces his long desire to dominate, he comes to enjoy spiritual happiness. In a Vedic mantra it is said that as he becomes learned in association with the Supreme Personality of Godhead, he proportionately relishes his eternal blissful life.
(Bhagavad-Gita----13:22----purport).

The Lord informs Arjuna that the conditioned souls can be divided into two classes as far as man's search for self-realization is concerned. Those who are atheists, agnostics and skeptics are beyond the sense of spiritual understanding. But there are others, who are faithful in their understanding of spiritual life, and they are called introspective devotees, philosophers, and workers who have renounced fruitive results. Those who always try to establish the doctrine of monism are also counted among the atheists and agnostics. In other words, only the devotees of the Supreme Personality of Godhead are best situated in spiritual understanding, because they understand that beyond this material nature are the spiritual world and the Supreme Personality of Godhead, who is expanded as the Paramātmā, the Supersoul in everyone, the all-pervading Godhead.
(Bhagavad-Gita----13:25----purport).