Depression can be a gift.
At least situational depression can be.
I am usually a fairly happy person, but I do get depression with almost any kind of major change. I've come to realize that, and accept that as part of my life. It has been as regular as clock work with every major life change. Graduate from nursing school, get depressed. Change jobs, get depressed. Get married, get depressed.
I also consistantly get depressed when I've been overly tired for long periods of time or simply overwhelmed with life.
Once I started to understand the patterns and causes for my depression, it was actually quite empowering. I get depressed when I feel like my life is out of control, either due to a major change or due to being overwhelmed and overly tired.
I have learned to use it a reminder that I need to slow down and take care of me. When depression shows up for me it means that I need to step back, look at my life and decide what is important and what is not. Sometimes it means that I need to cut something out, sometimes it means I need to add something. It motivates me to take control of the situation I am in and make deliberate decisions to make changes in my life.
(I do want to be clear that I am speaking specifically about situational depression. There are several kinds of depression, some of which are serious medical conditions and are not related to one's situation. If you have depression that does not resolve with making positive changes in your life, I would encourage you to see a medical professional for evaluation and treatment.)