I've encountered it many times in my life, with the people I love and work with. Do you struggle with depression yourself? If so, you should know that it is an extremely common *disease*, and often requires change and treatment. There is no known cure for depression, but it should be treated.
It's sad that depression is often overlooked, and considered insignificant. It takes cases like Robin William's suicide to raise interest in it, and then people slowly forget again. Nonetheless, I encourage anyone, and everyone struggling to get help. Sometimes help starts with someone just willing to listen. :-) Don't let depression segregate you, as you are not along in this big spinning ball that we call Earth.