I understand what you are trying to say, I think.
Sadness is an emotion and all people feel sadness. Therefore, all people experience depression.
However, it is different...It lasts longer and affects more areas of one's life. It doesn't go away without lots of work and it can come back for apparently no reason.
Also, depressed persons still have down days too.
Depression isn't just a down day, a period of sadness or of withdrawing. It is more than that. It is feeling sad for no reason, crying for no reason, wanting to stay in bed for maybe a day or maybe a month--or having trouble sleeping more than an hour at a time, wanting to hide away because we feel like a burden on others, not eating, not cleaning, not attending to hygeine...It overwhelms, it takes over, it hangs on, it prevents a person from having a peaceful and fulfilling life.
Meds are an option. Some do and some don't. Talking is the way to find out how you are thinking and how that works or doesn't work for you. Thinking is what drives us to elation or depression so it helps to understand how we think. That is what therapy does.
just my 2 cents..