There are different factors that lead to depression, that's well known. It doesn't occur just like that without some triggers and there are things we say and do that don't help. Bullying, abuse, and loss are factors that cause people to fall into depression. Depression is not just explained by a chemical imbalance. People experience it differently, for different periods of time. Therapists don't like people thinking of it as illness. And others think of it as to do with one's attitude, negative ways of thinking, percieving and acting in situations.