Well, here's something to consider when thinking about spontaneous emission. If you have an electron in a high-energy state, then intuitively you understand why it would emit a photon and fall into a lower-energy state: It's natural for systems to want to lower their energy, in the same way that it's natural for water to want to flow downhill. But this intuitive answer doesn't actually make any sense, by itself. When an electron emits a photon, its energy goes down, but the energy in the electromagnetic field goes UP. The total energy is unchanged. So the real question is not: why does the electron's energy go down, but why does nature prefer to give its energy to photons, as opposed to electrons?
Well, we can understand that through entropy, which amounts to counting states. There is only one (or a small number) of ways that a bound electron can absorb a quantity of energy, because there are only a few states associated with a given energy. In contrast, there are infinitely many ways that photons can absorb a quantity of energy, because there are continuum-many photon states. So if you pick a way to split energy up between an electron and the electromagnetic field, it's overwhelmingly more likely that most of the energy will go to the electromagnetic field. So what we observe is that electrons tend to radiate their energy away.
Now, if the electron itself has continuum-many states, then this counting argument doesn't apply. Now, there is no good reason, as far as entropy, for the electron to give up its energy to the electromagnetic field.
