Anyone who knows basic Chemistry knows that a molecule likes to be in its lowest energy state, called the "ground state". We can then speak of an "excited state" as merely a state of greater energy.
When excited, a molecule gets "jumpy" and starts tossing its electrons around more vigorously. But it doesn't simply throw them arbitrarily.
Like Juggling, it's a semi-controlled act that nature somehow finds a way to properly balance; you must throw the balls to a certain precise height to juggle well - electrons must reach only a certain precise energy level for the molecule to remain stable . A picture illustrates those precise energy levels.
Of course, get too excited (throw the balls too high), and you lose your state (juggling FAIL). With molecules, promoting electrons to too high an energy level increases the chance of losing them.
If a molecule loses an electron, we have a free radical) on our hands. These free radicals, having lost that electron, try to get it back, and tend to be more reactive as a result (maybe stealing electrons from other molecules).
Now, compare this to a frantic seller in the stock market. It's almost as if nature wanted chemistry to function the same way at every level of granularity!
Fortunately, we humans aren't brainless molecules. We can and we must separate excitement from emotion. Lest we become free radicals.