For example, when they ask, "How's it going?". You want to respond, "Lousy," or "shitty," or "Well, I'm not dead" because that pretty much sums it up. But then they want to ask you what's wrong, and you really don't want to get into it, or you don't even know how to explain what's wrong with you. So you respond with a "Good" or a "Fine," both of which are lies, and they really don't mean anything. Communication failure.
Or they ask, "How's your family doing?" Now, you don't talk to most of your family, and some of them you actively hate, and the rest are in and out of the hospital or just as depressed as you are. You don't want to talk about it, and they don't want to hear about it. So, of course, you say they're doing "Fine."
Or maybe they ask, "How's your job?" and you want to reply, "Same shit, different day," or "It sucks." But you know they don't want to hear you complain about your job; no one wants to hear the constant stream of negative thoughts running through your head. So you tell them the job is "Okay" or that you're just glad that you have a job in this shitty economy.
It doesn't work. While you're busy trying not to say all the negative things that you're thinking and only saying one or two words, you clearly can't keep up your end of the conversation. Sometimes you forget to ask them the same questions, and then there's a lull.
When you go through this process enough times, you don't want to do it anymore. You avoid people and isolate yourself even more, which - surprise! - increases your depression.
I think Allie at Hyperbole and a Half understands: