Why did Hashem make for example the human body so complex? Example... 37.2 trillion cells in each person. With a million cells I'd be dumbfounded.. He didn't need to go to 37.2 trillion. Moreover, I will never understand the million cells, yet alone the 37.2 trillion cells in each human.
A famous answer I have heard (as mentioned in the comments above) is that the complexity of the human body is a definitive proof that there must be a G-d. Only Hashem could create so many complex elements within the body, such that when a person stops to think about the incredible minutiae of each organ and the role that it performs it can only prove that it is of Divine creation.
I can't find the piece by Rabbi Tatz but I remember him speaking specifically about the human eye and how wondrous it is. Rabbi Avigdor Miller zt"l is recorded as saying in Rav Simcha Bunim Cohen's Rabbi Avigdor Miller Speaks, Vol. 1, Mesorah Publications, p.184 the following;
...so when a human being is finally born, it is a bundle of nitrogen from the air and oxygen from the air and carbon dioxide from the air. Only they are recombined in such staggeringly complicated chemical combinations that it is a neis.
But this is not what the ma'amar, Shelosha shutafim yeish ba'adam, is telling us, because you could say the very same things about a rabbit. This ma'amar means something entirely different. It is telling us that although the miracles in the creation of every living thing are beyond the ability of the human mind to fathom, a human being is something far greater than that; he is a miracle that is far beyond the miracle of life. It is a special manifestation of the Presence of Hakadosh Baruch He that makes this miracle happen.
All these myriad cells are made up of just three particles: Protons, neutrons and electrons. Permutations, replications, combinations and rearrangements give you the apparent diversity and complexity. All the laws of nature we figured out, that square with our observations, are simple statements, yet they give rise to complex structures. From simplicity, complexity. The lesson: God is One.