I think the reason why it may come back again and again or occur for long duration for some people is that they haven't got to the core issues and tackled them, and haven't made necessary changes. They still have things blocking them, keep getting into similar repetitive negative unhelpful patterns. And, there are suppressed bottled up deep emotions /feelings still. We're not always conscious of things we do. We can do things that don't help us subconsciously, without much awareness. Our thoughts can make us feel negatively when they are automatic or impulsive. Dwelling on /focusing on negative stressful unpleasant things leads to more difficult feelings.