Martin Seligman was one of the earliest psychologists to believe in the idea that depression is at least partly cognitively based, ie. its about how you think and what you think about. In at least one way, this is true. Depressed people tend to be unable to differentiate between small, manageable events and large, unmanageable events. Partly they do this by linking everything to their sense of self-worth, and generalize up to it. As an example:
Say you drop something, a normal person would be like, “I dropped it” and pick it up without a second thought. A depressed person might go, “I dropped something. I always drop things. That means I’m clumsy. That means I’m a useless person that can’t do anything right.”
In the same vein, it is hard to give constructive criticism to people like this. “You don’t listen” translates easily into, “You’re a worthless person”.
All this happens in a fraction of a second. Pessimists also tend to think in similar ways. Some people become way to attached to school in this way, seeing each grade that they get as a reflection of their intelligence, and thus their worth as a person. It is best to avoid this form of thinking.
Another tip that comes from this thinking is that if you do have to give constructive criticism to someone (whether they are perfectly healthy or not), be sure to praise some quality, deliver the criticism and praise another quality (called the criticism sandwitch, picked that up from the 4 Hour Work Week). Another important thing is to be specific to an instance, and do not imply a weakness of character or such. Suggest simply that in this one instance, that person might have done something that may not have come upto expectations, or may not be best for them. Avoid getting personal and generalized statements like, “You never listen.” Or, “You’re always clumsy.” Suggest instead that, “Last week when we were talking about something important to me, I felt that I wasn’t being listened to well. I know you were listening and that you care, but I feel most listened to when you do x, y and maintain eye contact with me.”