Arjun became so good because of his exceptional focus, enthusiasm, and eagerness to learn more!!

The Vrishnis and the Andhakas, and princes from various lands, and the (adopted) son of Radha of the Suta caste, (Karna), all became pupils of Drona. But of them all, the Suta child Karna, from jealousy, frequently defied Arjuna, and supported by Duryodhana, used to disregard the Pandavas. Arjuna, however, from devotion to the science of arms, always stayed by the side of his preceptor, and in skill, strength of arms, and perseverance, excelled all (his class-fellows). Indeed, although the instruction the preceptor gave, was the same in the case of all, yet in lightness and skill Arjuna became the foremost of all his fellow-pupils. And Drona was convinced that none of his pupils would (at any time) be able to be equal to that son of Indra.

Clearly, Guru Drona gave same instructions to all, yet Arjun became foremost of all and excelled in lightness and skills.

Again, Guru Drona tried to be partial towards Aswathhama, but with his hard work and devotion, Arjun did not let it happen!!

"Thus Drona continued giving lessons to the princes in the science of weapons. And while he gave unto every one of his pupils a narrow-mouthed vessel (for fetching water) in order that much time may be spent in filling them, he gave unto his own son Aswatthaman a broad-mouthed vessel, so that, filling it quickly, he might return soon enough. And in the intervals so gained, Drona used to instruct his own son in several superior methods (of using weapons). Jishnu (Arjuna) came to know of this, and thereupon filling his narrow-mouthed vessel with water by means of the Varuna

weapon he used to come unto his preceptor at the same time with his preceptor's son. And accordingly the intelligent son of Pritha, that foremost of all men possessing a knowledge of weapons, had no inferiority to his preceptor's son in respect of excellence. Arjuna's devotion to the service of his preceptor as also to arms was very great and he soon became the favourite of his preceptor.

He tried to teach Aswathhama more by sending other pupils away, but Arjun with his devotion started filling vessel with Varunastra and returned early to learn alongside Aswathhama!!

Once again, Guru Drona tried to restrict diversity in Arjun’s learning but Arjun is Arjun!

And Drona, beholding his pupil's devotion to arms, summoned the cook, and told him in secret, 'Never give Arjuna his food in the dark, nor tell him that I have told thee this.' A few days after, however, when Arjuna was taking his food, a wind arose, and thereupon the lamp that had been burning went out. But Arjuna, endued with energy, continued eating in the dark, his hand, from habit, going to his mouth. His attention being thus called to the force of habit, the strong-armed son of Pandu set his heart upon practising with his bow in the night. And, O Bharata, Drona, hearing the twang of his bowstring in the night, came to him, and clasping him, said, 'Truly do I tell thee that I shall do that unto thee by which there shall not be an archer equal to thee in this world.'[1]

Even though Guru Drona tried to restrict Arjun, Arjun with his devotion impressed his preceptor and forced him to admire his learning capability and hard work and hence make him the greatest.

"Two of Drona's pupils became very much accomplished in the use of mace. These were Druvodhana and Bhima, who were, however, always jealous of each other. Aswatthaman excelled everyone (in the mysteries of the science of arms). The twins (Nakula and Sahadeva) excelled everybody in handling the sword. Yudhishthira surpassed everybody as a car-warrior; but Arjuna, however, outdistanced everyone in every respect--in intelligence, resourcefulness, strength and perseverance. Accomplished in all weapons, Arjuna became the foremost of even the foremost of car-warriors; and his fame spread all over the earth to the verge of the sea. And although the instruction was the same, the mighty Arjuna excelled all (the princes in lightness of hand). Indeed, in weapons as in devotion to his preceptor, he became the foremost of them all. And amongst all the princes, Arjuna alone became an Atiratha (a car-warrior capable of fighting at one time with sixty thousand foes). And the wicked sons of Dhritarashtra, beholding Bhimasena endued with great strength and Arjuna accomplished in all arms, became very jealous of them.

The instruction was same for every student, but Arjun was the only warrior who became Atiratha (the highest ranking of any warrior) that too only in gurukul!!

Most of them were skilled in one field, but Arjun surpassed all in every respect!

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