Please realize you are not alone in this. Many many and I say again, MANY people suffer through depression throughout their life. It's normal and can be anything from situations in your life, food/diet, hormone imbalance, etc.
I myself have struggled on and off for many years. I have skeletons in my closet and it took me along time to sort out my feelings and find the best way for me personally to deal with it.
As you, when the world is fine around them still find things just not right. There are helpful tools out there. Could be as simple as medicine from the doctor, changing your diet, your excercise routine, etc. Or...possibly there is some underlying issue that is buried deep inside that needs attention.
Just don't feel alone or isolated and realize there is so many help options out there okay?
