Sadness is a very normal and useful emotion. We can't avoid sadness. It's part of the natural ups and downs in life. However, sometimes if sadness feels like it's really pervasive and taking over, it may be a sign that there is a depressive disorder going on. Depression disorders have many different features and there are different types. However, what they all have in common is that there is a fundamental inability to function in life's roles, so when sadness is taking over and you're unable to perform at work, be present in your relationships or it's impacting the way you feel about yourself, that may be a sign that it's time to seek treatment, as sadness has become a depressive disorder.