You have said it all, most depressed people have the biggest smile, they are very good at hiding their feelings and desperation, some even party a lot and have fun but internally they are battling with depression. Mostly Depression is not really about struggling with financial stability sometimes people battling with depression just feel something is missing in their life, in most cases they don't know what that is, they just miss the absence of the unknown.
The saddest thing about depression is, 90% of people battling with depression don't even know they have it because it is not a disease or some plague you could identify with physical symptoms.
In addition to the symptoms of depression you have listed, based on research, most depressed people also suffer from anxiety, panic attacks, sexual identity crisis, post traumatic stress disorder, inferiority complex and self denial.
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Yes. All listed are some exterior factors that lead to a case of depression. Depression in my opinion happens easily and as you pointed, is not easily discovered.
Depressed individuals may have the wrong idea or they just like to believe they are okay. It's sad that the world is no longer kind to people.
The world has never been fair, from my own perspective. It is left for people like you and I to assist depressed individuals in whatever ways we can.