As mentioned below, Nicotine and caffeine are useful to plants, and to humans. Potatoes, eggplants, tomatoes, tobacco all have nicotine. Humans just happen to have receptors for the chemicals in our brains/nervous system.[0]<p>interestingly, i was going to look for plants that hit our cannibinoid receptors and found this:<p>> Myrcene is found in extremely high concentrations in hop oil, making up to about 75% of the extracted volume in some varieties, and is also found in high levels in mangoes, lemongrass, thyme and verbena.<p>Myrcene is a terpene which is a "cannabimimetic" compound;<p>> B-caryophyllene is found in black pepper, cloves, rosemary, hops, caraway, oregano, basil, lavender, cinnamon, and many more plant species. In most of these species, β-caryophyllene is a major constituent of the essential oil.<p>Flax (linseed) contains, possibly, straight cannibinoids (in fact, CBD! 2012 discovery). Also, curiously, at least to me, all grasses humans consume are partially (marginally?) converted to compounds that interact with humans' opioid receptors. It's one of the reasons [some?] humans get withdrawal symptoms if they go on a diet that reduces sugarcane sugar, wheat, etc.<p>I'm repeatedly reminded that everything on earth is connected, even if only by heritage and geographic proximity to our own evolution.<p>[0] Ketchup has niconoid and opioid compounds in it and that's probably why i go through about a half gallon a month ;-) and combined with french fries it's a potent nicotine lunch with a side of opioids.<p>Also i'm a layman. My facts are true, but i sometimes use the wrong words because i haven't drilled the exact terms forever, because people's eyes glaze over if you start saying "nicotinic acetylcholine receptor agonist."