Everyone gets depressed or feels melancholy from time to time.
Whether one believes in God or not our lives are for the most part the end result of the choices and decisions we have made along the way.
All of our actions are preceded by our thoughts. What you fill your mind with clearly affects how you think as well. Focusing on what you don't have instead of what you're striving for can also lead to depression.
Cutting yourself from family and friends or otherwise (isolating) yourself from people and activities can cause depression.
You stated: "I have been the most nice, sincere person, anyone could ever know...." Are you an extrovert or introvert? Do you show an interest (in other people)? or Do wait to see if people will take an interest in you?
Are you "living life" or are you a "spectator"? Are there things you enjoy? Are you dating or in a relationship with anyone?
Having goals keeps you focused. The only cure I know of for depression is taking action.
Try to "unplug" from negativity for a weekend. Don't watch or listen to the news, don't read news or inflammatory articles online. A lot of folks go to bed with negative things on their mind and wake up tuning into the news to start off with more negativity. If something pops up avert your eyes or change channels. Watch DVDs, go for a walk, listen to your favorite upbeat music, see a comedy, sign up for one of the hobby groups on Meetup.com. Spend time with those whose company you enjoy, read biographies of people you admire, self-help and inspirational books, create a journal describing your life the way you want it, what you'd do, and where you'd go...etc
Always have something to "look forward to" on your calendar. It can be the release of a new movie, book, an event coming to your town, a party, a vacation or whatever. People who have (nothing to look forward to) are likely to get depressed. For them today is like yesterday and tomorrow will be like today. Make a list of the things you want to do and get busying trying to accomplish those things. Life is for the living!
Lastly if you truly believe you have a chemical imbalance or suffer from (clinical depression) then you should go see a doctor or therapist.