system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 - Why do medicines from plants like the poppy "fit" humans so perfectly?

I've curious as to why certain plans such as the poppy have parts such as morphine,  codeine etc that affect humans so specifically.

Why do we possess such a specific ability to be be receptive to these plants?
gpt
Many of these chemicals have a purpose to the plants.  Usually to keep certain animals or pest away so they don't eat them.

Caffeine is a mild pesticide, but since we are so much larger than insects it doesn't kill us.  Nicotine is along similar lines.

Capsaicin in spicy peppers really only effects ~~manuals~~ mammals.  So it stops many ~~manuals~~ mammals from eating those fruits.  Instead birds eat them and spread the seeds farther.