Hey. I have had the same problems sometimes. Think about your life, and who you interact with everyday. Maybe it's a problem with your family and you just don't want to admit it.
I used to feel that way because of my boyfriend. We are still dating, and he is such a sweet and sensitive guy. Sometimes it bothered me because he wasn't 'the best looking guy' but he was too nice to break up with, i'd feel horrible.
Maybe you are feeling depressed for a reason you thought might be small, or not noticable, you just need to look closely and find out, just like I did. I learned to get over the fact that my boyfriend isn't the best looking guy, and I could do better, but he is the sweetest in the world and I know I want to stay with him longer.
Good luck and I hope this helps a little!
-jody