Question
How strong lord krishna and Arjuna’s friendship was?
Answer
Krishna & Arjuna are believed to be the pair of Nara & Narayana. It's briefly discussed in this question. Their friendship stands immovable on the test of time.
Needless to say that Arjuna always regarded Krishna in the highest regards. IMO the reason he got to hear the Gita was, he surrendered himself to Krishna (without knowing his Vishwaroopam yet)
Now I am confused about my duty and have lost all composure because of miserly weakness. In this condition I am asking You to tell me for certain what is best for me. Now I am Your disciple, and a soul surrendered unto You. Please instruct me.
Not that he never considered Krishna as his friend!
BG 11.41-42
Thinking of You as my friend, I have rashly addressed You “O Kṛṣṇa,” “O Yādava,” “O my friend,” not knowing Your glories. Please forgive whatever I may have done in madness or in love. I have dishonored You many times, jesting as we relaxed, lay on the same bed, or sat or ate together, sometimes alone and sometimes in front of many friends. O infallible one, please excuse me for all those offenses.
During Mahabharata, I would refer below text, showing how Krishna felt about Arjuna:
After Ashwatthama killed all UpPaandavas, Krishna describes an old conversation to Yudhishtira. In which Ashwatthama wanted the Sudarshan Chakra (Discus) from Krishna, but doesn't succeed. Krishna asks Ashwatthama the intention behind it and here how it goes ...
...At this, Drona's son became filled with sorrow. After he was tired with the exertions he made, he ceased, O Bharata!
When he withdrew his heart from that purpose, I addressed the anxious and senseless Ashvatthama and said, "He who is always regarded as the foremost of all human beings, that wielder of gandiva, that warrior having white steeds yoked unto his car, that hero owning the prince of apes for the device on his standard, that hero who, desirous of vanquishing in a wrestling encounter the god of gods, the blue-throated lord of Uma, gratified the great Shankara himself, that Phalguna than whom I have no dearer friend on earth, that friend to whom there is nothing that I cannot give including my very wives and children, that dear friend Partha of unstained acts, never said unto me, O brahmana, such words as these which thou hast uttered...
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