You can't hide your personality in aikido. Whoever you are, it all shows up on the mat.
I was ruminating on this last night after class as I was partnered with a particularly, shall we say, "headstrong" fellow. He's a nice enough guy, but he's a bit difficult to have a conversation with. Very opinionated on even the weather.
We partnered up last night and it was really difficult. He's stiff and muscles through every technique. I've long past the point where guys like that discourage me, however. I know what I'm trying to do and how I want to go about learning it.
During the first year of my practice, I would get very discouraged by such things. I would think, "Oh, great, my aikido doesn't really work unless the guy cooperates..." But I have since learned that is a very wrong way to look at it.
It isn't really aikido when one is using brute force to execute a technique, even if that works with less experienced uke. A higher level yudansha can reverse such muscling technique in an instant.
No, to perform an aikido technique properly, one has to be relaxed, centered and have the ki flowing well. However, this is very hard to do in the beginning stages. So one's partner should try to follow a properly executed technique. There are two good reasons for this. 1.) It allows nage to really practice properly. 2.) It also allows uke to be more responsive. This, in fact, makes uke more of a threat as he can adapt to changing conditions and keep his balance to a much greater degree. This aspect is often overlooked.
An uke that hunkers down with the attitude, "Go ahead, try and move me now." Is responding to the simulated situation in an incorrect way. Would anyone really ever attack like that? Of course not. He'd get demolished with a strike to the head or something.
Aikido is about handling and redirecting real attacks, not about wining a judo point or anything else. My point being, it is to uke's "benefit" as well as nage's if uke remains responsive and gives a line of attack.
And I realized I shouldn't be discouraged if a "non-compliant" uke "messes up" my technique. Someday, I'll be at a high level and I'll be able to move these guys, without muscling, whether they want me to or not. Until then, I'll appreciate my partner allowing me to practice and doing the same for him.
I am beginning to learn how to handle these guys, too. I don't let them dictate to me how I practice my aikido. Just because they resist, I won't get tense! I am learning to just slow up and relax and let my center get involved. Sometimes it actually works! ;)