What kind of explanation would there be for someone who is not reasonable?

Emotion probably has a lot to do with it. If I’m feeling particularly emotional or stressed out, I know I’m much more likely to behave irrationally.

That being said, I’m generally a pretty rational and reasonable person, but my opinions and reactions can be really influenced by stress/emotion.

When people are unreasonable, words aren’t going to work as well. You have to make them feel while they’re wrong. That often means introducing them, face to face, to those who are the victims of their unreasonably-held beliefs.

I agree with @Seelix that emotion probably has a lot to do with is. In addition to emotion, it could also have to do with their beliefs and how they were raised. It will vary from one situation to the next, so it’s hard to judge.

You need to be a bit more specific, I think, about what brand of “unreasonableness” you’re talking about.

For example, for one spouse to give the other “the silent treatment” and not discuss a grievance is one type of “unreasonableness”. For people to have the various religious beliefs that they do—in spite of clear and ‘reasonable’ scientific evidence to the contrary—is quite another kind of “unreasonableness”. (This time I’m not knocking religious beliefs, merely using them to illustrate a point.)

This unreasonable person has probably been listening to too much country music.

Change their channel and carry on.

How someone acts usually seems perfectly reasonable to them, even if it doesn’t to you.

Some people are unreasonable because they are extremely angry and can’t calm down. Extreme anger can be caused by all sorts of things from discovering infidelity, to having been cheated out of money, to not getting expected treatment or services that were paid for (or not) to getting laid off from one’s job.

Some people are unreasonable because they are ignorant and unable or unwilling to see facts.

Some people are unreasonable because they are ill (mentally or physically) and simply cannot do what is considered reasonable. Some of these folks might want to be reasonable but simply can’t because of circumstances going on inside their minds or bodies, and others in this group may not be able to distinguish between what is reasonable and what is not.

Some people are unresonable because they are mean or selfish and don’t care about anyone else except themselves. For them it’s more convenient to do what’s easy or enjoyable than to do what is reasonable, even if it causes harm to themselves or others.

Part of the pain and difficulty in dealing with unreasonable people is trying to figure out which group (or others that I haven’t even considered) that they belong to. Figuring out the motivation for the un-reasonable-ness (is that a word?) is the first step in dealing with/working with/working around/avoiding/tolerating/putting up with/putting a stop to or running from the behavior.

I give policeman, doctors, soldiers, therapists, teachers and parents a lot of credit for having to deal with this phenomenon on a daily basis.

Lack of anger management.