Oftentimes this occurs when we're unhappy with some facet of our lives. We're aware there was nothing major that changed in our life from yesterday to today. However yesterday we were in good spirits and today we feel depressed.
Each of us has our own "triggers" that bring us down or shifts our focus on things we don't have instead of looking at all the great things we do have. Sometimes bad weather that locks us indoors can bring us down, having a major disagreement with someone we love, being stuck in traffic for hours, having an unexpected bill pop up for repairs, watching too much negative news on TV, interjecting ourselves into other people's problems. Feeling we lack control is depressing.
Keeping things in perspective is the key to living a well balanced life. When we accept the fact that this NORMAL to feel this way from time to time and we KNOW it's going to pass then we don't keep feeding our mind with more negativity. 'This too shall pass.' is our mantra.
Once we learn to recognize it when it seeps in we can make a (conscious decision) to do something we enjoy or spend time with people we enjoy, read an inspirational book, listen to our favorite music, go see a good movie, take a walk and people watch, do anything that takes the focus off ourselves for awhile.
Depression takes hold when we choose to cut off the rest of the world, only look at the negative things, and withdraw into ourselves. The only cure I know of for depression is taking action.