I think the reason people want the majority of people around them to be similarly-minded is because the majority will ultimately sway the politics of the area/country, and that will eventually manifest itself in rules and laws that do affect everybody.
Speaking only for myself, I'll say that I do feel more comfortable when surrounded by mostly similar people (in terms of broad culture, I'm not talking about race, although race is one of many external signs that may indicate cultural inclinations). Within a mostly homogenous-ish mass, I'd feel 'at home' enough to be happy exploring the differences of resident minorities without feeling threatened by some sort of total conversion or assimilation.
I like differences. I like that people are different and cultures are different, and I think that we need protective bubbles around groups to a degree to protect those differences, but that we also need mingling and communication and mixing so that we don't all become entirely rigid and stagnant.
I think we need a balance of "different" and "same" in order to experience the flavours of both.
Within that line of thinking, I like to think of myself as having a 'homebase' which is made up mostly of people who share a common parent culture, and a small sprinkling of people who are "other" but welcome and desired as bringing something fresh and different.
However, I think things have been mismanaged in the UK to the degree that people feel like their 'homebase' is under threat, and is becoming so changed that it is no longer a pleasant sampling of something different, but complete conversion.
Ideally, I'd like to see a global country where we literally do all think of ourselves as a worldwide community containing pockets of 'protected-and-happy different communities', much like the human body contains organs that are all different but work together. But thats a whole other debate. 