I have written about gratitude before. It is something we often forget to acknowledge when we see others with more than we have or when situations do not turn out the way we would have liked. It's easy to focus on the negative; it's so glaringly obvious. Positive things can be so much more subtle, like good health or a perfect train connection.
Perhaps you've heard of the Law of Attraction. The short version is this: energy flows where attention goes. If you focus on the negative, you get more of it. If you focus on the positive, you get more of that. It's important to recognize and be grateful for the good things so that the universe knows we want more of that.
I have a friend who puts the Law of Attraction to work for her by making a list of all the things she is grateful for every night. The list can include everything from a great cup of coffee to an abiding love for her husband. While I occasionally take stock, I don't remember to do this nightly. It's a good habit to take up.