People outside of science often want to know "but why?" or "how exactly? what is the mechanism? what carries the force?" . At first this seems like a reasonable question, you don't just want to have these rules, you want to know where they come from, what they mean exactly. But if you think a bit more, it should be clear that these questions are absurd.
Let's say you know the fundamental physical laws. These are expressed as mathematical rules that tell you the behavior of objects. Say for example we lived in a world with only Newtonian dynamics and gravity and that is all the laws. Someone asks "but what *is* gravity exactly?". I ask : How could you ever know? What could there ever be that "is" gravity? If something was faccilitating the force of gravity, there would have to be some description of that thing, some new law to describe it. That would mean some new rule to describe this thing that carried gravity. Then you would ask "well where does this rule for the carrier of gravity come from?" and you would need a new rule. Say you said "gravity is carried by the exchange of gravitons" ; then of course they could ask "why is there a graviton, what makes gravitons exactly, why do they couple in this way?" etc.
The fundamental physical laws cannot be explained by anything else.
That's almost a tautology because that's what I mean by "fundamental" - you take all the behavior of the universe, every little thing, like "I pushed on this rock with 10 pounds of force and it went 5 meters per second". You strip away every single law that can be explained with some other law. You strip and strip and finally you are left with a few laws that cannot be explained by anything else. These are the fundamental laws and there is no "why" or "how" for them. In fact the whole human question of "how" is imprecise; what we really should say is "what simpler physical law can explain this phenomenon?". And at some point there is no more answer to that.
Of course this is assuming that there *is* a fundamental physical law. Most physicists assume that to be true without questioning it, but I wrote here at cbloom.com long ago that in fact the set of physical laws might well be infinite - that is, maybe we will find some day that the electrical and gravitational force can be explained in terms of some new law which also adds some new behaviors at very small scale (if it didn't add new behaviors it would simply be a new expression of the same law and not count), and then maybe that new law is explained in terms of another new law which also adds new behaviors, etc. ad infinitum - a russian doll of physcial laws that never ends. This is possible, and furthermore I contend that it is irrelevant.
There is a certain human need to know "why" the physical laws are as they are, or to know the "absolute" "fundamental" laws - but I don't believe there's really much merit to that at all. What if they do finally work out string theory, and it explains all known phenomena for a while, but then we find that there is a small error in the mass of the Higgs Boson on the order of ten to the minus one billion, which tells us there must be some other physical law that we don't yet know. The fact that string theory then is only a very good model of the universe and not the "absolute law" of the universe changes nothing except our own silly human emotions in response to it (and surely crackpots would rise up and say that since it's not "100% right" then there must be angels and thetans at work).
What if we found laws that explained all phenomena that we know of perfectly. We might well think those laws are the "absolute fundamental" laws of the universe. But how would we ever know? Maybe there are other phenomena that can't be explained by those laws that we simply haven't seen yet. Maybe those other phenomena could *never* be seen! (for example there may be another entire set of particles and processes which have zero coupling to our known matter). The existance of this unexplained phenomena does not reduce the merit of the laws you know, even though they are now "not complete" or "don't describe all of nature".
It's funny to think about how our intuition of "mass" was screwed up by the fact that we evolved on the earth in a high gravity environment where we inherently think of mass as "weight" - eg. something is heavy. There's this thing which I will call K. It's the coefficient of inertia, it's how hard something is to move when you apply a certain force to it. F = K A if you will. Imagine we grew up in outer space with lots of large electrical charges around. If we apply an electric field to two charges of different K, one moves fast and one moves slow, the difference is the constant K. It's a very funny thing that this K, this resistance to changes of motion, is also the coupling to the gravitational field.