I'm going to disagree with a lot of people in this thread. A lot of aikido techniques, even when done properly, do inflict pain on the uke. That is not to say that goal of the nage ought to be to inflict pain, but I would think there is something wrong if I spent half-an-hour practicing sankyo and felt no pain at the end of it.
I am still very much a novice, so you can take my words with a grain of salt if you like, but I think pain is an essential element of the martial arts. The unique spiritual benefits offered to us by the martial arts are rooted in the opportunities they give us to face our own fear, insecurity, and pain. If we discard these opportunities, we might as well be practicing yoga.
Aikido techniques shouldn't cause pain... Aikido practice does cause pain. There is a reason it is called "practice", not "I got it right the first time".