Why are the theories of physics so similar in their structure?
There are a number of possibilities. The first is the limited imagination of physicists: when we see a new phenomenon, we try to fit it into the framework we already have—until we have made enough experiments, we don't know that it doesn't work. So when some fool physicist gives a lecture at UCLA in 1983 and says, “This is the way it works, and look how wonderfully similar the theories are,” it's not because Nature is really similar; it's because the physicists have only been able to think of the same damn thing, over and over again.
Another possibility is that it is the same damn thing over and over again—that Nature has only one way of doing things, and She repeats her story from time to time.
A third possibility is that things look similar because they are aspects of the same thing—some larger picture underneath, from which things can be broken into parts that look different, like fingers on the same hand. Many physicists are working very hard trying to put together a grand picture that unifies everything into one super-duper model. It's a delightful game, but at present time none of the speculators agree with any of the other speculators as to what the grand picture is.

The various approximations that constitute our current physics theories are successful because simple mathematical structures can provide good approximations of how a self-aware substructure will perceive more complex mathematical structures. In other words, our successful theories are not mathematics approximating physics, but mathematics approximating mathematics!

[M]athematicians and physicists think alike; they are led, and sometimes misled, by the same patterns of .

In the early 1990s, I came to suspect that the quest for a unified theory is religious rather than scientific. Physicists want to show that all things came from one thing: a force, or essence, or membrane wriggling in eleven dimensions, or something that manifests perfect mathematical symmetry. In their search for this primordial symmetry, however, physicists have gone off the deep end, postulating particles and energies and dimensions whose existence can never be experimentally verified.

In the absence of experimental evidence, basic beliefs of theoretical physicists may initially have almost a religious flavor, guided by faith and aesthetics. Fortunately unlike religion, these beliefs soon face the hard test of experiment.

We have to realize that a unified theory of the physical world simply does not exist. We have theories that work in restricted regions, we have purely formal attempts to condense them into a single formula, we have lots of unfounded claims (such as the claim that all of chemistry can be reduced to physics), phenomena that do not fit into the accepted framework are suppressed; in physics, which many scientists regard as the one really basic science, we have now at least three different point of view (relativity, dealing with the very large, quantum theory for an intermediate domain and various particle models for the very small) without a promise of conceptual (and not only formal) unification; perceptions are outside of the material universe (the mind-body problem is still unsolved) - from the very beginning the salesman of a universal truth cheated people into admissions instead of clearly arguing for their philosophy. And let us not forget that it was they and not the representatives of the traditions they attacked who introduced argument as the one and only universal arbiter. They praised argument - they constantly violated its principles.

What we realized is that... these theories are generic to a huge class of systems that have these particular very unstructured, underlying rules. ...[P]eople have been struggling for a long time... How does general relativity, the theory of gravity, relate to quantum mechanics? They seem to have all kinds of incompatibilities. ...What we realized is at some level they are the same theory!

Physicists describe the two properties of physical laws—that they do not depend on when or where you use them—as symmetries of nature. By this usage physicists mean that nature treats every moment in time and every location in space identically—symmetrically—by ensuring that the same fundamental laws are in operation. Much in the same manner that they affect art and music, such symmetries are deeply satisfying; they highlight an order and coherence in the workings of nature. The elegance of rich, complex, and diverse phenomena emerging from a simple set of universal laws is at least part of what physicists mean when they invoke the term "beautiful."

The fundamental materials from which we construct our picture of the universe may appear in different shapes, but there is really very little discontinuity between what seem at first sight very different views.

By having simplified what is known, physicists have been led into realms which as yet are anything but simple. That at some time, they, too, will appear as simple consequences of a theory of which no one has yet dreamed is not a statement of fact. It is a statement of faith.

[T]he universe, we may rest assured, is all alike, the intricate and the simple. It is all a universe of law, from the daisy to the star and from the diatom to the philosopher, from the flowing rivers and growing fields to the processes of our own brains.

to avoid what he calls physics envy, the common human craving to reduce enormously complex systems (such as those in economics) to one-size-fits-all Newtonian formulas. Instead, he faithfully honors Albert Einstein’s admonition: “A scientific theory should be as simple as possible, but no simpler.

If one was sometimes led astray by trying to simplify the elements of a science, it is because one has established systems before putting together a fairly large number of facts. Some assumption, which would be very simple when one considers only a class of phenomena, requires many other assumptions if one wants to leave the narrow circle in which we had initially withdrawn. If nature has offered to produce the maximum effect with minimum causes, it is in all of its laws that it had to solve this major problem. It is without doubt difficult to discover the foundations of this wonderful economy, i.e. the simplest causes of phenomena considered from such a wide point of view. But if this general principle of the philosophy of physics does not lead immediately to the knowledge of truth, it can direct the efforts of the human spirit, by leading it away from theories which relate the phenomena to too many different causes, and by adopting preferably those based on the smallest number of assumptions, which show to be more fruitful in consequences.

How does it come about that we have been able to find satisfactory hypotheses to explain electricity and magnetism, light and heat, in short all other physical phenomena, but have been unsuccessful in the case of gravitation?