Is there a practical evolutionary reason why we get high off certain plants or substances?
Because they cannot run away or fight back physically, plants have developed static defences against their predators. These include physical defences such as thorns and hard outer coverings, and chemical defences. The chemical defences are usually targeted against the main predators. In some cases, that is animals, so the plant or is fruit is poisonous to animal (and a surprising array of food plants are poisonous unless properly treated – kidney beans, manioc are two examples). But in most cases the main predators of the plant are insects. So the plants generates substances to deter insects, whose metabolisms are quite different from ours. One of the most effective ways of chemically attacking any animal is to attack the nervous system (hence the fear of the uses by terrorists of nerve gasses). So these chemicals are quite likely to attack the nervous system. Of course, a dose which will kill an insect making a main meal of a plant, or even living on it, will have only a small effect
