You know, I've always kind of wondered the same thing. My diagnosis is MDD, and I'll get horrible days and then on occasion I'll get happy days where I'm able to be my happy, normal not-depressed self. But honestly, I think that my diagnosis is right, because my happy days are not like "woohoo, I'm indestructible! And everything is awesome!"

So, yes. Depression does have ups and downs. Life has ups and downs. But it's when those ups are far far above and beyond the normal "happy" that it starts to qualify as bipolar.