Melancholy. No real reasons for it; just a time of sentiment and deep thoughts. But if it lasts longer than the day and proceed to depressive and gloomy mood; it may be the onset of Depression. That is the time to seek out a trusted person (with an opened mind), someone you knew who genuinely care and will understand (not necessarily family members or close friends) and form the thoughts and feelings that you have into words. It helps to see yourself from another's point of view. If their feedbacks does not help and the sorrow begins to deepens into a desperate need you can't identify or a sense of despair that lingers, it's time to seek a good counselor.
If you can't find someone who you can open up immediately; don't wait. Try to write the thoughts and analyze the roots of the thoughts yourself. Write a positive words next to the negative ones. Look at the positive words, you can find yourself in the positive too. There is two images. During times of depression, the negative impresses itself. Bring back the positive. Normally, the reason you can't seem to identify it with yourself; is because you do not believe it. Stop that thought pattern immediately. It is not the truth.