What we focus on, we get more of. It's a simple fact. If you are busy looking for the wood, you will miss seeing the beauty of the trees.
Us humans seem to be programmed to focus on things which we perceive as a threat or which are not happy or joyful to us - maybe it's a survival trait? Our heads are often full of unpleasant past events, present worries and future concerns. This puts our body on "high alert" - adrenaline and cortisol are released, our digestion shuts down, our hearts beat harder or faster. We all know that feeling and lots of us are spending way too much time in that state. Long-term it can have a very detrimental effect on our health. We are supposed to get into that state when it's needed (ie, when we're facing an actual threat) and then get out of it and back to what should be a 'resting and digesting' state.
One of the best ways to get out of the 'fight or flight' state on a long-term basis is to create a gratitude list and focus on it. A gratitude list can be a written list, sharing verbally (recorded or with others) or even a collection of photos. It contains everything in your life which gives you joy and/or which you love. It can be as small as a wild flower you spot on a walk or as big as the love you feel for someone. It will probably grow and grow as you keep adding to it and your gratitude will become a healing state you can access quickly and easily. Like most new habits, all it takes is practice.