Feeling hopelessness or believing one has no options for improvement are things that cause one to feel depressed. If you believe tomorrow is not going to be any better than today or next year will not be any better than this year it's so easy to conclude that life is not worth living.
Depression at it's very core is a "self-centered" activity. Depressed people spend the bulk of their time only thinking about (themselves) and their situations. They are usually not (active) or busy living life. Very often (they) cut themselves off from others, decline invitations to parties and outings, refuse to answer the phone, or check emails...etc And then they proclaim they're "alone" or "no one understands me". Well what can you expect if you cut people out of your life? Of course you're going to be alone!
In other instances being an "introvert" or not making an effort to form friendships or new relationships causes people to feel depressed. They "unrealistically" expect the world to come knocking on their door to connect with them. When it doesn't happen they become "wallflowers" in the party of life.
Depressed people often believe things "happen to them" and their (actions) (choices) or (decisions) have no bearing on their life. They convince themselves that on a planet containing 7 Billion people that they are the only one who have faced these same struggles. When you believe it's "only you" then you're bound to sink into a depression.
The only real cure I know of for depression is "taking action".
Taking action initiates feelings of "self-empowerment" within us.You have to get outside of yourself and look at your options, study people who have faced similar issues and found a way to thrive, stay in contact with people, and even help others who are doing worse than yourself, or simply be a friend to someone, join a few groups at Meetup.com who share similar interests or have hobbies you enjoy.
Always have something on your calendar to "look forward to"! It could be the release of a new movie you want to see, an upcoming concert/play, a vacation destination, party or event. It's good to have at least one thing per month on your calendar.
People who have "nothing to look forward to" become extremely depressed.