Homo sapiens sapiens are the product of billions of years of evolution starting with one celled organisms and leading to astounding levels of complexity. Our species' current level of success indicates our flexibility and cunning at responding to the multitude of challenges presented by an environment composed of a nearly infinite set of variables. Thus it seems somewhat ludicrous to suppose that the totality of the human psyche can be boiled down to various expressions or repressions of the sexual desire. However, if we take a Darwinian standpoint on the "purpose of life", namely to survive to reproductive age and then to reproduce, we can see that Freud may have been neglecting only one other major piece of the puzzle, fear, and in particular fear of death (to help facilitate the survival part).

In my opinion fear has had an impact on human culture at least as important as sexual desire, providing a massively persuasive idealogical basis for religion (heaven, afterlife, immortal soul, etc. to help ease that annoying fear of death), as well as supplying the impetus for the cohesion of small groups into larger societies (easier to protect your camp, kill that mammoth, etc. with 20 guys instead of 2, increases rate of survival, helps quell fear of death).

Thus, while I'm not trying to insinuate that Freud never considered fear of death, because I'm sure he did, it seems odd to me that he should omit it in describing the emotional and psychological growth of little kids. He certainly references fear as a symptom of repressed sexual urges, but I find it unlikely that latter is always a cause of the former and never the other way around.