One wakes up one year and realizes you did not produce as good a set of races as the year before.
I also feel it usually takes 4 or 5 races to hit your best marathon for your body to be accustomed to the training AND the race itself.
Lastly, races bring a lot of energy into your focus on running, and are (usually) a good time.
You aim to win the same races you won before, but your time is done; there is someone who is hungry, really hungry.