Hi,
A good question which attracts interest because the detailed rigorous science of it is so obscure to most of us
Firstly (as I understand it) the speed of light as an absolute is an intrinsic property of space-time. It is the natural and inevitable speed of other types of elementary particle besides the photon, for instance bosons.
Prof Brian Cox (calm, calm, ladies
) has co-written a terrific book which has in it the most simple and elegant explanation of why c is such a basic property of space-time, "Why does e=mc^2 and why should we care". It is really worth a read.
These quasi-particles are "mediators" of different fields, that is, they make the fields known and felt to us. The photon is the mediator of all kinds of electromagnetic field effects. The undiscovered graviton would be the mediator of gravitational fields, if we could find one
Photons are without any mass at rest, so have no inertia, and then need no force to push them along to c. That speed is actually their natural state, in a vacuum anyway (they go slower in a housebrick). Indeed they are really only spoken of as particles at all because they behave that way in certain circumstances. At other times they behave as a wave. This is one of the planks (no pun) of Quantum Theory.
I've given myself a headache now, so I'll leave the rest to some real experts.
Cheers