You don't. They won't understand. No one who hasn't been truly depressed can fully understand. You might get a T to try and explain it to them, that way at least it's coming from a professional and they might stop asking simply out of embarrassment (that they can't acknowledge that they didn't understand what the T said )

Depression is a medical illness. When the term "imbalance of chemicals in the brain" is used, it means that either the brain isn't making enough of the normal happy chemical you need, or that the brain isn't using the chemical that is being made.

Why do you feel it's your job to educate them? Ask them to review the information and help YOU understand why you feel so sad...to help you in that way.

Depression tells us lies. That's part of why you feel everything is your fault. Like you would feel this way instead of happy, if you had a choice? I don't think so, do you?

Tell them to ask a doctor to explain depression to them. Hang in there.