Among the liver's many jobs is to detoxify substances within the body. When the liver discovers a toxin (which are generated within the body by our own metabolism, as well as coming in from outside of us), it chemically modifies it. The goal is to change the toxin so that it becomes both inert (i.e. no longer toxic) and easily excreted (usually through the kidneys).
The liver is quite good at this. But sometimes it gets confused. On occasion, the liver sees a molecule that's actually inert, but it treats it like a toxin. It chemically modifies it as usual, and in some cases, the liver actually creates a toxin out of a perfectly harmless substance. In other words, the liver doesn't always detoxify; sometimes, it toxifies.
The mind seems to be capable of a similar reaction. Have you ever had a loved one say something completely innocent, something that would sound harmless to most people, but been suddenly thrown into anger, or guilt, or grief? Your mind has taken a nontoxic thought, and created a toxic one out of it.
We can't control the reactions in our livers, but our minds are different. In the moment, we may not be able to stop the underlying process through which the strong emotion was generated. But we can see it for what it is; a toxin created from a harmless idea. We can be curious about what triggered that reaction. In this way, maybe we can get a little distance from that toxin, and maybe we can even interrupt the process so that it isn't so strong the next time.