Plants reduce stress. We have an innate desire to be connected with nature. Unfortunately we increasingly find ourselves inside artificially lit buildings and have very little contact with the natural environment. This can increase feelings of stress and anxiety.
What's comforting to know is your house plants, local public parks, or private gardens are full of soil which contains microbes dubbed 'outdoorphines,' because they work as natural anti depressants and stimulates natural 'happiness chemical,' serotonin in our brains.