Most people just don't know what depression actually is. They think it is just sadness.
It is more than this. A sad person can switch to happy within a few minutes, depending on what is happening in their environment (maybe they were sad because they lost their dog, but found it and got happy and relieved). A depressed person cannot switch from depressed to not-depressed nearly as fast, as there are physical and chemical changes to the brain that need to change before the depression begins easing. I don't know how to magically balance neurotransmitters instantly; do you? I don't know how to fix a brain.
When people are clueless, take the opportunity to educate them. Point out the physical, chemical, solid problems behind depression; explain why it is different from merely being "sad" and why it might take a while to repair yourself and heal.
