1. I don't think it's helpful to categorize people. Humans will be varying degrees of reasonable/unreasonable for a variety reasons and constantly in motion on that spectrum. It's more productive, for both selfish and altruistic reasons, to try to understand the why of their behavior.
2. Getting that understanding is hard. Especially over mediums that leave out non-verbal communication. But it's really worth your time to discern if there is any area where you've communicated poorly, so you can improve, AND it's really worth your time to recognize when a person has already made up their mind and not waste anymore effort trying to change it.
3. Even when a person has made up their mind beforehand, it might be worth your time to try to understand where they're coming from, depending on whether you are in a position to simply ignore them or not. If you can't ignore them, digging deeper can open up your options to get buy-in, or lead to workarounds. Of course, very relationship, and situation dependent.