This is why article 370 was created

Article 370 is one of the most controversial provisions provided to Jammu and Kashmir, drafted under part XXI of the Indian Constitution. This Article provides special immunity to J & K and grants special autonomous status to the state. Article 370 was created as an instrument to accede the state of Jammu and Kashmir to the Indian Union.

From where this problem of article 370 arises? Let us dive deep into history. It was the time when the transfer of power was happening i.e when we were about to be accorded freedom. On 20th February 1947, His Majesty’s Government announced that independence would be given to British India, the plan was to create two independent dominions of India and Pakistan based on the Hindu and Muslim majority areas of the various provinces of  British India. Policy framed by Britishers announced that with the withdrawal of paramountcy, the princely States would become ‘independent’. However, neither the Cabinet Mission nor the British government thought about the future of 562 independent princely states. The idea of 562 states exercising their own power was too dangerous and chaotic, civil wars were on the card, and maybe that is what the Britishers would have wanted.

V P Menon, Secretary to Sardar Patel’s States Department, came up with a suggestion that princely rulers should be persuaded to hand over just three functions of their States to the central government: defense, external relations, and communications — the same functions which had been always exercised by the British government. The persuasion and aggression of Sardar Patel merged all princely states into the Indian Union except the small State of Junagadh and the two large states, Hyderabad and Jammu & Kashmir. Junagadh and Hyderabad acceded to the Indian union through efforts of the Indian government, and the army was used to accede Hyderabad.

The situation was different with  Jammu and Kashmir- the largest of the three. A Hindu ruler, Maharaj Hari Singh, was ruling over the majority of the Muslim population.  Pakistan soldiers along with Pathan tribesmen invaded this state (Brief History of India Pakistan wars over Kashmir), creating chaos and spreading disturbances. In a state of panic,  he signed the agreement and accede to India, but unlike others, he refuse to accept the constitution of India. He insisted that, as provided by clause 7 of the Instrument of Accession, it was not committed to accept the future Constitution of India. Instead, it decided to have its own separate State Constitution. 

This stand of Jammu and Kashmir raised a major legal hurdle before the newly created nation, and hence, the Indian Constituent Assembly was compelled to make a special provision to cover the particular and exceptional case of Jammu & Kashmir. And that special provision is what we call today 'Article 370', which is a hot topic of discussion in political and social circles of India.

Edit: Article 370 was abrogated by the Indian Government on 5 August 2019.