Life is a succession of events, positive and negative situations, that will influence our way of thinking and behave, but also the way we feel. Sadness often comes from a loss or an experience we identify as negative, or simply because we haven't fulfilled our desires. This sadness can be transitory, lasting hours, days or even weeks, but when it persists over time and changes the way we feel, think and act, we may be dealing with a bigger problem: depression.