It's really fucking suspicious that mushrooms evolved mechanisms to produce serotonin.

But it helps when you remember that a mushroom is the fruit of a (usually) much larger organism. Then you can start applying normal fruit rules. Some want to be eaten, or picked up and moved around. Some want to keep insects from infesting the fruit. Others don't give a damn and release spores into the wind or water.

Also remember that nicotine is an insecticide. Insects that nibble on tobacco die, which prevents infestation at scale. (Un?)fortunately it's also neuroactive in apes, so we farm incredible quantities of tobacco to extract its poisons.

There is no logic in evolution at large scales. Things happen, sometimes there's fourth order effects like some oddball internal hormone causing wild hallucinations in apes. It's all random optimization for small scale problems that ripple out to unintended large scale consequences.