I actually don't exactly agree with the above, as it implies that the uke is on purpose trying to make it difficult for the nage to execute the technique. To me, it is important that the nage (if in position to do so) or sensei show at some point to uke that there is always an appropriate response to any attack (quantitative & qualitative) but it is equally (if not more) important to understand and question why the uke performs this way. The easy way is to say (or write on a thread) that he is a jerk and stop thinking about it. But there might be some more noble reasons why uke behaves like that.
So toss out the word "jerk" if it seems too judgmental to you (an understandable reaction, obviously). I found it interesting that in context, the person who first used it (I thought) made it clear that he was using "being a jerk" in a somewhat different sense than one would usually use it. I think I understood what he meant, but if you can't see it (again understandable), then set the term aside. Likewise "obstinate attitude", if that strikes you as too judgy. I think it's just being used as shorthand, though. I think everyone who's trying to explain this to you understands quite well that this is not just a case of uke intentionally being difficult out of sheer meanness or stubbornness or (insert character flaw of choice here). Sometimes uke is hopelessly uncoordinated and so unknowingly does something that puts him/herself in a really bad position. Sometimes uke has a "bright idea" that really isn't so smart at all, resulting in (again) a really bad position. Sometimes uke is inflexible or out of shape and can't move in ways that are required to receive the technique safely. Whatever. But the important thing is that in all cases, uke is doing something that impedes his/her own learning.
I came to aikido with experience in other martial arts. At first, much of what I saw made no sense to me. Many things were counterintuitive to me. But they also didn't contradict what I'd learned before. They were different, sure -- but looking back on my prior training, I couldn't point to anything that said, "Doing so-and-so (what I was learning in aikido) is pointless and stupid and will get you killed." So while I did examine what I was learning from a perspective of prior training, I didn't try to make it fit into that mold. If I'd been in a karate dojo, what I was learning would have been considered some pretty peculiar karate -- but I wasn't in a karate dojo. So, I didn't try to judge it by karate standards.
There's an inherent difficulty in a beginner trying to evaluate the quality and validity of what they're learning, whether it be aikido or physics. If you refuse to take anything on faith, even to keep an open mind to the possibility that what your teacher says is valid, and you insist on proof before you will accept it, you've created a dilemma for yourself. You demand an explanation before you are willing to accept the teaching, but you lack the experience and knowledge to understand the explanation -- and without accepting the teaching, you won't get it. You have to be able to accept at least the possibility that what you're told as so, and to practice accordingly, before you can gain an understanding of why this is so.
This is not to say that you have to accept anything you're told. You can always try to teach yourself, by experience and primary research. That's a pretty inefficient method, to say the least, as you often won't get much of anywhere. There are some things you really need a teacher for. You can also use your own judgment of character as to whether your teacher is a person of integrity or not. There is no formula, it's a matter of painful experience, but if you know how to spot a con, a manipulator, someone who is lying to him/herself or others...well, you know 'em when you see 'em. I've got a pretty good bullshit meter -- I trust it, and it has been a long time since it proved me wrong. So when I came to my aikido dojo, I was able to judge that the senseis and the students were people of integrity, sensible people, not deluded and not interested in deluding others. The practice made no sense to me at all. But because I felt that the teachers were trustworthy, I was able to maintain an open mind, to do things that made no sense to me, over and over again, and get the data points so that now they start to make sense, I can understand the explanations or explain them to myself. Without the data points, though, the theory would mean nothing to me. And without the open-minded practice, I'd never have gotten the data points. If I'd insisted on proof before practice, I'd never have gotten anywhere.