Assertion: The Pakistani leaders thought the Kashmir region ‘belonged’ to Pakistan.
Reason: Kashmir shared a very long border with Pakistan.

Both the Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.

Before 1947, Jammu and Kashmir (J&K) was a Princely State. Its ruler, Maharaja Hari Singh did not want to merge either with India or Pakistan but to have an independent status for his state. The Pakistani leaders thought that Kashmir region ‘belonged’ to Pakistan, since the majority population of the State was Muslim. But this is not how the people of the state themselves saw it— they thought of themselves as Kashmiris above all. This issue of regional aspiration is known as Kashmiriyat. The popular movement in the State, led by Sheikh Abdullah of the National Conference, wanted to get rid of the Maharaja, but was against joining Pakistan.