This is largely due to the fact that most people have an outdated and very incomplete idea of what the term "genetic" means. In high school biology class I was taught that our DNA hold the instructions for making us, encoded in a series of nucleotides (As, Ts, Gs, and Cs). Our "genetic code" is seen as a kind if instruction book that says "make this person this tall, with this color hair and eyes, and with this personality". It is seen by many as something that is "used" in the beginning, when we are growing as a fetus, and that results in unchangeable characteristics in us. This is actually quite far from the truth. An excellent article about epigenetics and how they relate to our behavior and health was recently published in the magazine Discover, which is a great place to start in understanding this perspective. You can also watch a narrated slide show from the science show NOVA about epigenetics here...another level or aspect of our genetics that is keenly responsive to our environment:
This is a quote from the webpage with the link to the slideshow "In this audio slide show, Dr. Dana Dolinoy of Duke University explains the role that the epigenome, a sort of second genome, plays in regulating the expression of our genes. As Dolinoy notes, we can no longer say with certainty whether genetics or the environment have a greater impact on our health, because the two are inextricably linked through the epigenome." Let me emphasize that last part..."we can no longer say with certainty whether genetics or the environment have a greater impact on our health, because the two are inextricably linked through the epigenome." The question that dominates media coverage of autism- the question of "whether the cause is genetic OR environmental" is moot. It is both, both are inter-related. There is another famous quote (please post in the comments if you remember who said it, I don't) is "genetics is stored environment".
The "reading" of your genetic code is not a passive process but an active process that is influenced by your environment, it is regulated by proteins that control which genes are turned on and when, in which part of the development process, and for how long. Many toxins in the environment including metals, pesticides, and chemical additives are known to effect this process, as are infections, nutritional factors, and even social factors. This quote from the journal article Prenatal environmental exposures, epigenetics, and disease explains this very well:
For more on the basics of epigenetics:
There is an episode of the tv show NOVA called "Ghost in Your Genes" that provides a good introduction to how epigenetics works.
The Power Of Genes, And The Line Between Biology And Destiny from NPR
The illustrated guide to epigenetics (from the magazine Mother Jones)
For very detailed and advanced information about epigenetics this lecture by Dr. Laura Landweber is very good "Epigenetics: Rewriting the information in DNA".
Also Science 101 for Parents: How Epigenetics is Revolutionizing the Understanding of Heredity which is a lecture presented at Rockefeller University that also gets into more advanced topics and also is more focused on the role of epigenetics in child development.