The electromagnetic force is one of the four fundamental forces. I am not a theoretical physicist, but I think it's safe to say that although we know a lot about how these forces work, we know little about why they work. I'm sure an expert in QED could go on for ages about your question, but my guess is much of it would be so abstract as to be of very little use to you. For chemist, even a physical chemist, charge (and its mediator particle, the photon) as a concept is accepted as an axiomatic fundamental reality. Everything else pretty much derived from this piece of information.