Homeostasis processes, and redundant gene-coding sites in our DNA, mean that your body is mostly robust to environmental onslaughts. Animals have evolved in a competitive environment where these problems already existed: parasites want to control your brain; predators want to envenomate you before they eat you; prey species want to stop you in your tracks with toxins. But life evolves to adapt to these problems, becoming able to shrug most of them off if it's a consistent part of their niche. Most of the "spices" we eat are plant toxins we've adapted to be fine consuming.

When mutations happen, they don't always immediately cause a problem. What they more-often do, is to cause the body (or in this case, the brain) to lose some of its homeostatic mechanisms — to become less robust to these environmental onslaughts. The affected system becomes more vulnerable, or loses compensatory capacity — it remains able to function in the normal case, but loses the ability to function in the edge case.

Of course, some of our body processes are more robust/protected than others. Some systems (reward; inflammation; sleep) are very easy to "hack", for some reason; while others (libido; fear) are nearly impossible.