Here's a good read regarding the oriental concept of the five elements and it's relationship to aikido.
The concept of the five elements, imho, is an extension of the Ying-Yang concept. Both are about the balance of nature. In ones journey of the aikido path, one would surely experience the different elements at different phases/levels of training. This is also a matter of choice of whether one choose to move from one phase to another. Ultimately, one man's Yin is another man's Yang.
Just my 2-sen.