When I grew up we kids were told that "you are what you eat". As an adult I am made up my own little slogan: "You are what you think". A mind filled with negative thought tends to make a person pretty depressed, and also has a weird way of making him focus on only the negative stuff that happens around him.
Getting anywhere with my healing process has taken extensive work to maintain a positive and optimistic frame of mind. Chronic pain often leads to depression, which only increases the perception of pain. Optimism isn't something I was born with, it was an active choice I took in my teens and have had to work continuously to uphold.
Anthony Robbins talks in "Lessons in Mastery" of how we should "stand guard at the door of our minds", and carefully filter what we allow to enter. Because we can choose our input, but once the information is in there we have to deal with it. I like to think that I have an actual body guard standing guard at the door of my mind, saying "Yup, ok" to things like Hyperbole and a half and "Nope, back off" to things like horror movies and catastrophe news. What kind of information would you like to fill your mind with? What can help you increase the joy in you life? What kind of input is harmful to you, and how can you protect yourself from it?