How about the analogy of an advertisement for Thomas' english muffins? I quote (not precisely):
"What makes them so good is all the nooks and crannys!"
I think of it (aikido or even life) sometimes as a road; wouldn't it be kind of boring if it was always on the same incline? I used to live in Colorado, and loved to just go for drives in the mountains. What made these drives so enjoyable was all the ups and downs and twists and turns in the roads, combined with coming around a turn or over a hill and seeing some astonishing scenery all of a sudden. Or driving across a valley or plateau straight toward an incredible mountain range or set of peaks, getting closer and closer. As you got closer, you would be able to see more and more details of the mountains, and then in a few moments, you would either be going over them or around them, and onto the next set of peaks or down into another valley, beginning the cycle all over again.
Anywho, just my two cents worth...