The training wheels have to come off eventually.
Taking to the open road (or in this case, grass) without training wheels on your bike is a freeing experience as a child. It's a rite of passage that's on par with the first time your parents let you stay home alone, or the first time you drive a car after obtaining your license.
So when this little girl successfully rode her two-wheeled bicycle for the first time, she was pretty pumped. Actually she freaked right out.