Many of the fundamental constants of physics and cosmological parameters appear to be fine tuned for the existence of complexity and life. This includes quantities such as the speed of light, the gravitational constant, the relative strengths of the strong and weak nuclear forces, the masses of the proton, neutron, and electron, and many others which are acutely critical to the stability of the universe and the various structures within it.
The mass of the proton relative to the neutron is critical to regulating the primordial abundances of the chemical elements in the initial creation of matter. Shortly after the Big Bang, the ratio of neutrons to protons produced was related to the mass difference between these particles. Had this ratio been only 1% smaller an equal amount of neutrons and protons would have been produced. (Instead of 1 to 8 in our universe.) An equal or greater number of neutrons would have resulted in a universe with all helium and no hydrogen. (Our universe is about 74% hydrogen.) No hydrogen means no stable long lived stars like the sun and no life which requires hydrogen. Now if this ratio was only 0.2% larger protons would decay into neutrons and there would be no atoms at all.[1][2]
The gravitational constant, the universe's density, and the cosmological constant are all balanced to a remarkable precision. Had any of these parameters been different by a tiny fraction of a percent, space-time as described by general relativity would be unstable resulting in either the universe's rapid collapse shortly after it was created, or the rapid dispersion of the gasses before galaxies could form. General relativity and big bang theory show that the universe is literally on a knife's edge. If it were not for this fantastic balance, there would be no galaxies, no stars, no planets, and no life.[3] Many other examples can be found in scientific literature.
This fine tuning can be seen as evidence that the universe was created by an intelligent being, as the probability of all the constants of nature having these particular values is very small. However, it can also be explained using the anthropic principle. This states that although unlikely, if the constants had any other values, life as we know it would not exist and so could not ask the question "why does the universe appear fine tuned for life?" Hence we should not be surprised that the universe is so friendly to life as we know it.