Recurring theme in most of the Partition stories
In almost all of the personal stories I have heard from my family and also watching the videos of Punjabis describing their lives in pre-partition Punjab, the recurring theme in their narratives is that all the communities lived in peace and harmony. They would attend marriages, festivals and nobody believed that these same Punjabis would then plunge into the depth of violence and hatred.
What caused this sudden change? In Punjabi the interviewer often asks "ik dum ay kyon hoya". There are no easy answers to this but there was an underlying sense of retribution or revenge that permeated in 1947. Initial riots in Bengal in 1946 and then in Rawalpindi in March of 1947 led to revenge killings in East Punjab and then counter killings in West Punjab simply created a cycle of violence that took a life of its own.
Political leaders on both sides were masters in instigating the people, telling them lies about how they are so different that they cannot live together. The rationale of two-nation theory forgot that there will be more Muslims still living in India than in Pakistan. But this ill-conceived rationale was enough to propel the Pakistan movement. For the Sikhs and Hindus living in West Punjab the idea of living in Pakistan surrounded by majority Muslim villages and towns was simply unacceptable. In United India they would have, in Islamic Pakistan it was out of the question.
Similarly the idea of Punjab partition ignored the fact that the millions of Muslims living in the Eastern half of Punjab (that included Delhi, and today's Haryana and Himachal) would be outnumbered and would face a difficult choice to either migrate or co-exist with their non-Muslim Punjabis. We are still today dealing with RSS led hate groups in India where lynchings of Muslims have become a common day occurrence. In Pakistan the social ostracization of the Ahmediyyas and Christians, the in-human blasphemy laws, shows how much that country has fallen in dealing with its minorities.
A United India would have tempered these hate groups. It would have tempered the tendencies of the fringe to compel one group to fight another. The national politics would have had stronger checks and balances to counteract any one community pursuing its own interests or discriminating others based on religion. National politics would have been more inclusive.