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.