Why do nearby charges increase the probability of virtual particles? I'm a layperson currently reading through Matt Strassler and on his website, he states, in relation to using positronium as an example of vacuum fluctuation effects:

The stronger force between the electron and positron makes it more common for virtual photons to be present

My background is based primarily on Griffith's Quantum Mechanics and Particle Physics texts, I also think I understand the basic application of simple harmonic oscillators as creation and annihilation operators.
I don't want to appear lazy, but of the several reasons I can think of to answer my question myself, I just don't know enough to say which of them is correct. For example, do the charges add energy to the field, or is there an interference effect between the electron and positron, if we treat them as wave packets? 
I know that the number of virtual particles increases as we approach the source, but how this is related to force in the way described is not immediately clear to me.
Obviously, I am as confused as I am ignorant on this topic, my apologies.
Is it possible to answer my title question, at the level of basic QFT, or to refer me to a source that does so?
 A: Quantum field theory is formulated in terms of fields, not particles. Particles are phenomena that the theory predicts when the conditions are right, but "virtual particles" refers to something else. 
Another page by Matt Strassler (https://profmattstrassler.com/articles-and-posts/particle-physics-basics/virtual-particles-what-are-they) hints at the fact that "virtual particles" are really just terms in a calculation, not discrete physical entities. The type of calculation that involves "virtual particles" is one that starts by solving the equations in the trivial case where nothing interacts with anything else, and then successively adding in more and more "corrections" to account for the interaction. The stronger the interaction, the more "corrections" are needed. These "corrections" are mathematical expressions that are often represented graphically as Feynman diagrams. Superficially, these diagrams depict processes in which the interaction between physical objects is mediated by one or more "virtual particles." However, what these diagrams really represent are mathematical expressions that arise in this particular method of calculation. 
Here's an analogy:
The words "carrying the $1$" refer 
to a step in the usual manual procedure
for adding two integers:
\begin{align*}
  \begin{matrix}
      \phantom{+}\scriptsize{1}\phantom{0} \\
      \phantom{+}25 \\
      +17 \\
\hline
      \phantom{+}42
  \end{matrix}
\end{align*}
"Carrying the $1$" is important 
for getting the right answer when the calculation is done
this way, but this isn't the only way to do the calculation,
and it's not a good way to think about the meaning of addition.
Similarly, the type of calculation to which the
words "virtual particle" refer is a type of manual calculation
that is mostly useful for describing scattering experiments,
where the interacting objects are close to each other only for a brief instant.
The longer the objects are close to each other, the more of these
"virtual particle" terms we need to add up in order to 
get a good approximation.
This is roughly analogous to the fact that when we're adding
two large integers by hand, we need to "carry more $1$s" than when
we're adding two small integers.
The excerpt from Matt Strassler's site, shown in the OP's question, is presumably referring to something like this.
There are situations in which a small number of these "correction" terms give an excellent approximation to the exact answer, and in those situations, there can be short-lived physically-observable particle-like phenomena that correspond relatively directly to the computational devices called "virtual particles". In general, though, there is no such correspondence.
A: Thinking more deeply (for me:) about the word force, I guess without a constant exchange of force carriers, self evidently there would be no force. Each force carrier (photons in this case) has an amplitude to act as a loop in a Feynmann diagram and produce virtual particles. 
Oh boy, I'm new to this.
A: Strassler says this in a section called “What would happen if $\alpha$ were closer to 1?” The fine structure constant is, roughly, the probability that an electron or positron “emits a virtual photon”. (In the mathematical expressions for the Feynman diagrams, each vertex contributes a factor of $\alpha$ to the probability of whatever process one is calculating.) So all he is really saying is that when you increase $\alpha$ you increase the probability of virtual photons. He isn’t comparing positronium to other bound systems like H atoms; he is comparing higher-$\alpha$ positronium to lower-$\alpha$ positronium.
