Put simply, depression looks and feels like giving up, and it is important to receive help precisely because of the damage we can inflict on ourselves when we are hopeless.
Just like anxiety, there is often a reason for depression. Sometimes, it is because a person was told by their caregivers that they were no good, and after a while, it just became easier to believe it. Other times, it is because things have not gone well in that person's life, either recently or consistently, and the person feels this must be their fault. Depression can even be a defense against rage toward others. If that rage cannot be expressed, it is often turned inward.
Popular depictions of depression often show someone who is droopy, sad, and tired. What is missing from these depictions is usually anger, anxiety and/or ambivalence. Depression can feel active and acute; it is only the outward presentation (which protects us) that is downtrodden. Depression encourages nonparticipation because we feel that by not participating, nothing bad can happen.
Working with depression involves returning to the beginning. What caused us to have such negative views of ourselves and the world? This uncovering process can take a lot of time. It can also be easy to confuse cause and effect. However, once the knot is untangled, a person can begin approaching their life with the outlook of someone not predetermined to fail.