Perhaps we need to define what that means, first of all. Here's the dictionary definition.
It's important to remember that there is an unwritten meaning here too, that what a person is intolerant of is disputable. All of us are intolerant of things that are obviously wrong/bad/harmful, for example genocide. The only people who tolerate it are those who would do it. There are no absolutes in ethics, but there are plenty of examples of wrongs that the vast majority of people would never accept. So, while the vast majority of people are intolerant of them, we don't classify this as intolerance.
This gives us a clue. Intolerance seems to be based on perceived harm. Makes sense. Why would anyone tolerate something harmful? It would be foolish, crazy even to tolerate something we know will hurt us. But when we talk about intolerance as an issue, as seen clearly from the dictionary (and remember, dictionaries reflect common usage) we are talking about a perception of harm that isn't obvious, or that others simply don't see at all. In fact, it could well be imaginary.
Nevertheless, that perception must be there. There has to be some motivation behind it. I think we all instinctively understand that this is what is going on, and we have done so for a long time. I have read a lot of 19th century literature, and there are plenty of examples of people being asked "but where's the harm in it?" when they object to something. It is a pretty standard response to any criticism - "it doesn't hurt you." In other words, you are being told to get over yourself.
I wonder how many people are that easily shaken out of it. Would it make a difference if it were a very silly, petty intolerance? Would that be easier or harder to rationalize?
I posted a test here a few days ago that examined implicit bias. Recently, research has been done with tests just like this that seems to show that racism is pretty much endemic in white people. Not surprisingly there are plenty of people who object to that idea. It's really not a very pleasant thing to accept. How much of this translates into actual intolerance is quite another matter. People who don't believe they are racist, or who are trying their very best not to be, may well be avoiding displays or acts of intolerance, so it is possible to suffer from implicit bias without it having any real effect.
It is even possible for a person to be openly racist in their words, while not in their deeds. Much of human society gets by in this way, we are basically pretty decent to one another, despite the horrible things we say.
And this applies to all areas where intolerance occurs.
As usual, I choose to use a silly example to demonstrate what I mean, because if I choose a real one, too much focus is placed on it. I want my example to be placeholder for all types of intolerance, not just one. Therefore I shall talk about the problem of hatism. Hat prejudice.
In the fictitious town of Examplis there is a problem with hatists. They are insisting that the new trend of wearing a green hat is a problem, because in their day everyone wore blue hats. Needless to say, the people who wear green hats are insisting on their right to do so, and saying there's no harm in it. The hatists know that they can't actually go around stealing people's green hats, and in any case that would only solve half the problem - they have no way of forcing people to wear blue hats.
So the hatists do the only thing they can do. They attack the green hat wearers every chance they get. They tell cruel jokes about them. They slander them. They don't employ them, or if they do they don't promote them, and they pay them less. They won't let them into their clubs. They won't rent to them. Hatists in the police force do random stop searches of people in green hats, and pull them over when driving for no reason.
Green hat wearers, along with other people who preach tolerance, become activists. They pass laws to prevent actual oppression of green hat wearers, and they make criticism of them politically incorrect. The hatists can no longer openly speak their hatism, but they don't actually change their belief that green hats are lesser, and therefore wrong.
What's missing from all of this? Communication. Freedom for Green Hats activists decide that they won't make the same mistake that intolerant people make. They must discuss it and try to find out what's behind it. Why do the blue hat wearers hate the green hats so much? But as soon as they show they are willing to listen, a group of the Blue Hat and Proud Of It people, complain about being persecuted. They claim people are being intolerant of their intolerance.
Nevertheless, an interview goes ahead and the blue hat wearer who represents them, says it's all a matter of a slippery slope. If you let people wear green hats, what next? No hats at all? Or perhaps people will wear penguins on their heads, or model tractors, or cheese sandwiches. When the interviewer suggests that perhaps even this would do no actual harm, the blue hat wearer started to yibble and rant about the downfall of society, then accuse the green hat wearers of trying to take over.
Silly. It sounds silly because it is silly. But can you smell the fear? It's all about fear. Fear of "other". Fear of change. Absurd fears of everything falling to pieces. Fear, in fact, of the End Of Civilization As We Know It.
That, I truly believe, is what lurks behind intolerance. There has to be something, and intolerance simply can't stand on its own. It must be motivated by fear. It's why media bias deliberately and systematically tries to create fear.
It's why threats work, especially vague ones. It's why propaganda is so incredibly powerful.
When an animal is frightened it attacks. Humans do too, but are often more subtle about it. Intolerance is a type of fear-based attack. It is a sneaky, back-door attack. It is, in fact, the action of a coward.