Plants have evolved to use any and all species to their advantage. We know that plants attract insects to use as pollinators. We know that plants use carbon dioxide that is exhaled by animals. We know that plants produce burs which attach to fur and feathers to spread seeds. We know that plants produce good tasting fruits and vegetables for all sorts of species to enjoy in order to spread their seeds even further. Plants live a lot longer, use less energy, and can survive drought and famine far better than most other life forms.
Have we considered that plants evolved to have certain effects on humans in order to ensure their survival? Plants with healing properties for humans tend to not go extinct. Humans tend to keep around plants that help them. When a plant has a healing property for us, we ensure its survival by continuously planting it and helping it grow.
Some plants such as tobacco have addictive chemicals in them to ensure their survival. The fact that tobacco contains an addictive substance has ensured that it is a hugely successful species; Humans have chosen to cultivate the plant because of the nicotine, and indeed make sure that weeds and pests may not interfere with its growth. Without the nicotine, tobacco would probably be considered to be just a weed and would not be nearly as successful. The original 'purpose' of the nicotine was probably to poison its natural enemies, which unfortunately it also does to its human cultivists.
Chris Wright, Twickenham UK
Some plants have the ability to get us "high" or to make us "trip". Marijuana is considered a weed, hence once of the names for it is weed. With marijuana legalization happening around the world, the plant is having exponential growth throughout the world ensuring its survival. Let's not pretend that marijuana being illegal ever did anything to get rid of it. It is a very popular plant and has been ever since it's discovery in early human history. Just because some people were afraid of a plant and made laws to try to stop it, didn't mean that they actually could. The hemp plant itself can be refined into a variety of commercial items, including paper, rope, textiles, clothing, biodegradable plastics, paint, insulation, biofuel, food, and animal feed. Hemp is so useful to humans, that it has ensured that as long as humans are around so the hemp plant will be also.
Plants smell pretty, look nice, have all sorts of properties that help humans and are soothing to be around. Humans believe that we are using plants when they are the ones using us. They've been around longer than we have, they know what they are doing. If they didn't want us to use them, they wouldn't let us. They would make themselves so unappealing to us that we would never want to go near them again. We know that Certain plants have innate pest-repelling properties. For example, research has found that some marigold species, catnip, and chrysanthemums contain phytochemicals that prevent insects (including mosquitoes) from feeding on them. If plants wanted to repel humans as well, they would find a way and it wouldn't even be very hard. Love and Peace!