Pakistan was established against the wishes of Hindus, so they never accepted Pakistan from the bottom of their hearts. Wonderful progress and stability of Pakistan constituted a major concern for them. So, they started launching aggressive actions against Pakistan.
The Kashmir conflict is the real cause of War 1965. India occupied Kashmir against the will of the Kashmiri people. Kashmiri people are in favor of accession to Pakistan, but India has always avoided holding the promised plebiscite in Kashmir by the resolution passed by the Security Council. As a punishment for supporting Kashmiri people morally and raising Kashmir issue all over the world. India imposed war on Pakistan in 1965. Pakistan extended moral support to the people of Kashmir and raised the Kashmir issue all over the world not like which India did.