This is my answer to the question "mistä teitä sikiää", which I hear every once in a while:
Multiculturalism is a very easy mistake to make - or at least was in the 80s and in the 90s. There are some people who encourage it for their own political purposes, but the reason the multiculturalism has become so popular with the educated masses is IMO simply that decent educated people (decent != educated, but there are a lot of people who are both decent and educated, and I am speaking about them) have a very strong tendency to know only other decent educated people, and have a chance to observe a perfectly peaceful interaction of cultures.
When you think of foreigners your think of your Spanish or Canadian or Sri Lankan or Arab coworkers; your friend's Chinese wife and another friend's Israeli husband; a Russian, an Indian and a Nigerian that you went to the university with; the kid of some Iranian dissidents who went to your high school; your father's Bulgarian or Portuguese coworker, etc.,etc. All of these perfectly nice people, getting along without any problems.
Of course people who come from different cultures can along very well. You can see it with your own eyes. People who claim otherwise - well, probably they just haven't seen it with their own eyes yet. Or are stupid. Or evil.
Since these people whom you know are all good people, and get along pretty well, their cultures must all be quite nice, and get along perfectly well, right?
Or maybe you don't believe that. Maybe your Bangladeshi coworker has told you about the war and how Muslims used to kill Hindus, or maybe your Cambodian friend told you about how Khmer Rouge used to kill, well, everyone. Or maybe even you yourself came from Russia and actually have a pretty good idea that the Western culture is much nicer than most or all others.
In this case, obviously, the people who come to the West came to the West because they preferred living in the West to living wherever they come from, and therefore like the West better. And of course they know better than to make the West into a semblance of their countries of origin.
Especially if you yourself are part of an immigration wave consisting of people who disliked their country of origin and were committed to the West, you assume that most non-Western immigrants are.
You might not care much for Islam, but you understand Muslim immigrants all the better for it: after all, who would ever want to live in a Muslim country?
Sometimes immigrants rob or rape, but of course so do the natives. Sometimes they clearly behave in bad ways that they have clearly brought from the country of origin, but these are Isolated Incidents (tm) and Exceptions from Which You Shouldn't Generalize.
And then you walk down Edgware road in London, in a Muslim neighborhood, and lots of people try to give you flyers calling you to Islam, and you are much annoyed, but not that much more than with Jehova's Witnesses or Hassidim.
And then you walk on Trafalgar Square the next day, and there is a demonstration, thousands of them, carrying slogans that say "Islam - the Future for Britain", and suddenly you realize that you really don't want Islam to be the future for Britain, or any other civilized country for that matter. Also, you find yourself thinking "if Islam is so wonderful, why don't you bugger off back to some Islamic shithole".
The same summer some things slowly connect in your mind. Khomeini's fatwa against Salman Rushdie, WTC bombing in 1993, an attempted attack on France with a plane hijacked in Algeria; the 1995 bombing of the Paris subway... a tiny minority of extremists, to be sure, but why always the same minority?
You start thinking suspicious thoughts about Muslims on your flights.
You read up on Israel and Palestine, which you have always assumed to be just a territorial conflict, and realize it's not just that. You find it natural that Palestinians hate Israel - after all, warring neighbors always do. What you don't, however, find natural, is that so do Pakistani and Indonesians, considering that they are neither neighbors nor at war.
And then you read an article where a Somali girl casually says that her parents would get angry if they found out she has read Anne Frank's diary, because "Jews are our enemies", even though there is probably not a single Jew in the whole Somalia.
And then your country gets attacked by a group of Muslim terrorists, and you see a lot of other Muslims celebrating. And then they attack your country's ally, and your country's another ally, and a place that just happens to be often visited by tourists from your country's another ally, and a lot of random Buddhists in Southern Thailand, just in case.
And then you check out the uneducated anti-multiculturalist rednecks and their writings again, and suddenly you realize that they don't mean every single immigrant, and suddenly they make a lot more sense than 10 years ago. They have changed. Or you. Or the world. Or all of the above.
You search for sensible Muslims. You find them, too. You also notice that they tend to receive death threats when they speak out.
You feel the world change around you. In the Western Europe where you moved just over 10 years ago there used to be no places where you'd be afraid to walk alone at night. Now there are places where you wouldn't go with a group of friends in the daytime. In the Western world of 1989, Khomeini's fatwa was taken as a joke, an insane demand by an insane old man. In 2006, there is a lot of otherwise normal-looking people who say "we should respect their culture" under similar circumstances.
You see huge crowds all over the Islamic world burn flags, embassies and occasionally each other because of a cartoon in the Islamic world's obviously most popular newspaper, Jyllands-Posten.
You listen to what various Muslims say. And you mostly don't like it.
You are receiving new information and adjusting your worldview. You don't like it either - but then the people who were unable to adjust their worldviews according to the new information did not live long enough to become your ancestors.
At some point you realize that the Exceptions from Which You Shouldn't Generalize are the ones with whom you do get along, not the ones with whom you don't. Even though the majority of the ones you meet in person are still very nice.
And then you see multiculturalists realize the same thing. Instead of bravely continuing with the "Isolated Incident" line, more and more of them start saying "we'll have riots on our hands" and stuff like that. They still sing the multiculturalis song too, but I wonder for how long anymore.