Depression is a very difficult state of mind.  When an individual is in a low mood, it can affect almost every aspect of his or her life - behavior, feelings, thoughts, and even well-being.  Do you know anyone who battles with this?
I do actually, i know many who struggle with depression personally. It is a truly devastating mental illness. 
I agree.  It runs in my wife's family.  Obviously, things like the loss of a loved one can trigger it; but there are so many causes, including physical, hormonal, or mental issues that can trigger depression.  Have those friends of your been diagnosed with clinical depression?