There's no clear explanation for why people who practice positive thinking enjoy the above benefits but researchers feel that having a positive outlook just helps you better cope with things and focus on solutions instead of magnifying problems. They're just better able to manage stress and setbacks than those who are more pessimistic. Also, people who are optimistic are usually more active physically and as a result, will tend to be healthier and live longer. They also tend to eat better, avoid alcohol, sugars and are non-smokers, all of which helps them enjoy better health.
In his 1967 paper, Arbitrary and Natural Reinforcement , Charles Ferster proposed classifying reinforcement into events that increase frequency of an operant as a natural consequence of the behavior itself, and events that are presumed to affect frequency by their requirement of human mediation, such as in a token economy where subjects are "rewarded" for certain behavior with an arbitrary token of a negotiable value. In 1970, Baer and Wolf created a name for the use of natural reinforcers called "behavior traps".  A behavior trap requires only a simple response to enter the trap, yet once entered, the trap cannot be resisted in creating general behavior change. It is the use of a behavioral trap that increases a person's repertoire, by exposing them to the naturally occurring reinforcement of that behavior. Behavior traps have four characteristics:
People who spend more time outside in nature have a significantly more positive outlook on life than people who spend a great deal of time indoors. Communing with the natural world increases people’s feelings of vitality and energy, and consequently has a large positive effect on their overall mental health. Being outside around trees and ornamental horticulture is proven to improve people’s mental health, and give them a more positive outlook on their lives. People who spend time outside every day are less likely to be depressed or stressed, and thus have fewer burdens on their mental health. (Barnicle 2003, Faber Taylor 2001b, Grinde 2009, McFarland 2010, Morikami Museum and Japanese Gardens 2009, Shoemaker 2009, Wolf 2004b)