It is easy to train in the spring when life is all around us and the weather is pleasant. It is difficult to train in the winter, when all we see is cold and dead. However, the Japanese have a saying, in the summer we learn technique, in the winter we learn discipline.
Physically, of course, this is true. In the spring your body is as supple as a young sapling, softer, so your technique improves. In the winter, with the cold, one muscle fights another, and one must force oneself to train. However, this is true spiritually, as well. In the spring our spirits are lighter and thus our movements become more subtle. In the winter, we must set our resolve to continue our training. Thus we learn discipline.