one bad week

i've been doing great with the running - at least 4x a week - until this week.
how is that i can have almost a month of greatness and then i fall off the wagon for one week and i feel god awful about myself.

the funny thing is next time i run i'll be faster and it will feel great - but right now i just feel terribly horribly guilty..

why is that i can have weeks of awesome performance but one bad week and i'm destroyed?