Physicists are constantly talking about how simple nature is. Indeed, the laws of nature are very simple, and as we come to understand them better they are getting simpler. But, in fact, nature is not simple. To see this, all we need to do is to compare our actual universe to an imagined one that really is simple. Imagine, for example, a homogeneous gas of neutrons, filling the universe at some constant temperature and density. That would be simple. Compared to that possibility, our universe is extraordinarily complex and varied! Now, what is really interesting about this situation is that while the laws of nature are simple, there is a clear sense in which we can say that these laws are also characterized by a lot of variety. There are only four fundamental forces, but they differ dramatically in their ranges and interaction strengths. Most things in the world are made of only four stable particles: protons, neutrons, electrons and neutrinos; but they have a very large range of masses, and each interacts with a different mix of the four forces. The simple observation we have made here is that the variety we see in the universe around us is to a great extent a consequence of this variety in the fundamental forces and particles. That is to say, the mystery of why there is such variety in the laws of physics, is essentially tied to the question of why the laws of physics allow such a variety of structures in the universe.
If we are to genuinely understand our universe, these relations, between the structures on large scales and the elementary particles, must be understood as being something other than coincidence. We must understand how it came to be that the parameters that govern the elementary particles and their interactions are tuned and balanced in such a way that a universe of such variety and complexity arises. Of course, one possibility is that this is just a coincidence. Perhaps before going further we should ask just how probable is it that a universe created by randomly choosing the parameters will contain stars. The answer, in round numbers, comes to about one chance in 10^229. To illustrate how truly ridiculous this number is, we might note that the part of the universe we can see from earth contains about 10^22 stars which together contain about 10^80 protons and neutrons. These numbers are gigantic, but they are infinitesimal compared to 10^229. In my opinion, a probability this tiny is not something we can let go unexplained. Luck will certainly not do here; we need some rational explanation of how something this unlikely turned out to be the case.
I know of three directions in which we might search for the reason why the parameters are tuned to such unlikely values. The first is towards some version of the anthropic principle.
One may say that one believes that there is a god who created the world in this way, so there would arise rational creatures who would love him. We may even imagine that he prefers our love of him to be a rational choice made after we understand how unlikely our own existence is. While there is little I can say against religious faith, one must recognize that this is mysticism, in the sense that it makes the answers to scientific questions dependent on faith.
A different form of the anthropic principle begins with the hypothesis that there are a very large number of universes. In each, the parameters are chosen randomly. If there are at least 10^229 of them then it becomes probable that at least one of them will by chance contain stars. The problem with this is that it makes it possible to explain almost anything, for among the universes one can find most of the other equally unlikely possibilities. To argue this way is not to reason, it is simply to give up looking for a rational explanation.
The Life of the Cosmos (1999), Lee Smolin: