Not all training rides are created equal

Why is that? With all the performance enhancement programs, training and exercise physiology, high tech training facilities, testing and expert coaches today don't you think  it should be possible to create and maintain the perfect training ride? The answer is clearly no. There are too many unknown variables that can impact any given training ride ranging from hours and quality of sleep, proper hydration and nutrition, energy level, wind conditions, temperature, muscle fatigue, motivation, not to mention the perfect playlist on one's iPOD. Some days you feel great and everything goes according to plan and others you can't wait for the ride to be over and feel like hanging up your bike for good.