[_bdog]

That is not too surprising. On the one hand serotonine not only acts as a neurotransmitter, but has a mechanical function controlling blood-pressure and a multitude of other things as well. (See this comment: https://news.ycombinator.com/item?id=9476594)

Easier triggered bodily alertness in unpleasant situations probably creates a feedback-loop which shuts off rational thinking (frontal cortex) further -> anxiety.

On the other hand, I think a good metaphor for it's complex role as a neurotransmitter is "bandwidth". If your serotonine-levels are too low, some regions in your brain have trouble communicating with each other. Hence when your amygdala starts sending out signals of imminence, your rational prefrontal cortex might be able to think "No need to be afraid.", but it's not able to properly communicate that back. The other way around works too: the signals of imminence might not arrive clearly ("I didn't even realize I was afraid"), so you can't act properly.

If serotonine-levels are too high, this communication and loops might work TOO well. Imagine a "weak non-specific signal" from a confused (lack of a better word) amygdala that starts off a thought, which in turn throws the amygdala into a feedback-loop that you can't get out off. In that situation it would have been easier if the weak signal would have been dismissed in the first place.