Waking up to a flat tire is one of my biggest let downs. It's along the same lines of waking up thinking it's Friday when it's really Tuesday, or biting into a warm chocolate chocolate chip cookie only to realize you used salt instead of sugar, or having a shower interrupted by a power outage.
You get up, planning the day in your head, looking forward to a brisk ride to wake you up on the way to work. You pull on your warm gear to brave the chilly wintry morning, stashing an extra layer or two into the messenger bag, checking the weather to know for sure. You plan your evening errands, making sure you have enough room for that last Christmas present you'll be picking up on the way home, and then pulling your gloves on you check your tire pressure only to be shot through the heart.
Then you'll get grumpy because you won't have been able to ride in today.