How and why did relations between hindus and muslims change over time?

Relations between Hindus and Muslims were largely based upon trade until around 900AD when outlying Hindu communities began converting to Islam. This precipitated some armed conflict and struggles for territory on the sub-continent. Much of Islam's spread was made possible through Sufism, a sort of hybrid of Islam and Hinduism. Integration occurred under British Colonial Rule, and both Hindus and Muslims banded together to resist the British. Things changed when the British partitioned India based upon religious demographics - creating Pakistan, Kashmir, and creating conflict between the groups that persists to this day.