Behind the division of the sub-continent lies a great debate, that of Jinnah vs Gandhi. Was it the true aspiration of the people to get divided geographically while getting independence from British rule? It seems the divide happened because of some ideology driven people (Iqbal) who convinced other leaders (Jinnah) to pursue it. Today, this is a raging battle in Pakistan, did Pakistan ever need to be an independent…
On a meta-level it's fascinating how the vast majority of countries in the world still tend to be mono-ethnic "nation states". Even in the most westernized parts of Europe there are overarching tendencies for nations to fly apart into their component sub-ethnicities rather than to stick together based on belief in common values. Look at Britain and the ongoing discontent of Wales and Scottland, let alone Northern Ireland. Or Belgium threatening to split into two countries. Or the Basques in Spain. Or the history of Czechoslovakia, the USSR, Yugoslavia, etc.