Why have certain plants evolved to contain psychoactive compounds?

Question

Plants such as marijuana (*Cannabis*) and kratom (*Mitragena speciosa*) contain compounds that affect the human brain. Why have these plants evolved to incorporate substances like THC and mitragynine into their leaves/flowers?

Answer 1

Most likely as a protective mechanism against other animals that want to eat such plants. Most, if not all of these psychoactive compounds have very bitter or otherwise undesirable taste.