system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does India and Pakistan hate each other?
gpt
The Indian subcontinent was ruled for a time by the British Empire.  After WWII, Britain relinquished its control of the territory.

The subcontinent is home to a wide variety of religions.  The two largest are Hindu and Muslim.  The two religions have a long history of struggle and there are centuries of bad blood between the communities.

As it became clear that Britain was going to give up control in India, a large group of Muslims decided that they would not be content to live in a Hindu-majority nation and they wanted a Muslim-majority nation as well.  The result was the partition of the territory, originally into two entities - India, the Hindu-majority, and Pakistan, which had two parts - one in the north, which is modern Pakistan, and another piece in the south which is now Bangladesh.

During the period of partition there was widespread disruption.  People who were Hindu were forced to leave Pakistan and people who were Muslim were forced to leave India.  Many many people died during this dislocation and there were bad acts on all sides.  It was incredibly traumatic.

After the dust settled the Pakistanis became convinced that the Indians were determined to reunite the whole subcontinent under one national government.  The Pakistanis have spent a disproportionate amount of money on their military to resist this.  The Pakistani military has run the whole country for long periods of time, to the detriment of civil society.  Pakistan and India have fought several wars over territory (and Pakistan had a civil war with the part of itself that broke away and became Bangladesh).  To this day one area, Kashmir, remains bitterly divided and disputed and is the scene of depressingly regular "incidents" between the two powers (and China, which also claims territory in the area).

Pakistan's powerful intelligence service has funded and trained terrorists who have made spectacular attacks against Indian targets - including the terror attacks in Mumbai a few years ago.  These links are just obfuscated enough that the international community can condemn them but not feel compelled to take more stringent action in response (to India's great frustration).

Over the past 50 years India has become a much richer society than Pakistan and that certainly contributes to the bitterness felt by the Pakistanis.  Within Pakistan there are rampant conspiracy theories about how and why this has happened, many of which essentially boil down to "perfidious interference by the Indians".

The combination of centuries of conflict, the trauma of partition, the continuing friction between the countries, and the disparate standards of living and security are the drivers of the hostility.