One major reason Pakistan and India want control of Kashmir?

For India, Kashmir became its integral part when Instrument of Accession (Jammu and Kashmir) was executed by Maharaja Hari Singh on 26 October 1947. According to which Jammu & Kashmir will be a part of India (with some terms & conditions), so naturally it should be India's part.

For Pakistan, Kashmir has become a Muslim dominated state, so naturally it should be their part as the whole partition of India & Pakistan was based on Religion (Hindu & Muslim).