The answer to this question probably lies in QFT, which I know just enough about to appreciate my current lack of understanding of the subject, if you follow me.
About a year ago I asked our Professor and was told:
a. "It's a property of photons."
b. To think in terms of inverse momentum, which sort of makes sense, provided you have the background.
So, in other words, is there an intuitive picture available to help understand the observed fact that two like electrical/magnetic charges repel and two opposite charges attract and a photon seems to "know" this and alter momentum values accordingly?
I do own a few QFT books, so if it's in page X of of say, QFT in a Nutshell, or Srednicki, that would be a great help.