When the Muslims arrived in India as conquerors, they were unable to convert the locals, just as the Buddhists and Christians would also be unable to do. The locals most likely to convert, of course, were the Shudras the common laborers, and the untouchables. So Hindus regard Muslims in the same way the regard the Shudras and untouchables: people who have stupidly denied that there is a cycle of rebirth in which the worthy die and are reborn as higher caste people. The resentment is like that of Americans have for people who crowd in line, except far greater.
The Muslims dislike the Hindus because of their denial that there is one God, that there is One True Faith, that there will be some sort of Apocalyptic End of Time, and their insistence that there is a cycle of reincarnation that goes on forever. And of course, the idea of caste is also alien to the Muslims, who consider (at least in theory) that all souls are equal. Indian Muslims also are divided into castes, but their castes have no standing among Hindu
To the Hindus, India is the center of creation, and it is illogical and anti-patriotic to leave India on pilgrimages to Mecca.
The traditional view of Hindus is that if one crosses the Dark Water (ie the ocean), one loses their caste entirely. Gandhi was born in Gujarat, then went to England for a law degree, then spent time in S. Africa. As such, he was already seen as a radical by a lot of Hindus, since he had crossed the dark water.
In South Africa, Gandhi's clients were wealthy Indian Muslims. There was also a large group of Hindu coolies who did menial labor. Gandhi saw them all as Indians, and united them to stand up for their rights as citizens of the British Empire. The result was the Brits stopped importing Indians to South Africa.
Gandhi was assassinated by a Hindu who hated his preaching that Muslims and Hindus should be legally considered as equals.
It is probably safe to say that Indian immigrants to the US are not typical Indians. Many might be the children of Indians who belonged to different castes and/or spoke different languages and communicated in English.
Castes are divided into subcastes, and then divided by region. There are over a thousand castes. Hence the immense popularity of matchmakers. It is quite hard to match up eligible couples otherwise.