I can honestly say that I know what you are talking about. I get that way often and dont know why. Even if everything in my life is going swimmingly, Ill just feel very depressed.
Yes, I believe you dont need a reason to be sad. Needing a reason to be sad seems just the way of modern society, needing an explanation for everything.
i feel like it hurts more when im sad and i dont know why.. it makes me feel confused.. which leads to anger.. and more sadness... i think confusion is the worst.. i guess im an analytical person.. i need a reason to be feeling something.. and i dont want to feel it.. if i dont know why.. though i do feel things and i dont know why, it sucks... hell im confused now..
Smile though. Its good for the soul.