I know I struggle with chronic, low grade depression. Maybe even major, but if it's all I know, it feels pretty normal to me. I feel rather emotionally flat much of the time. Not unhappy. It runs in my family, and my daughter has had some severe mood issues. She is fine on a SSRI. Still the occasional swings, but she IS a nearly 16 yo girl!
Exercise, sunshine and a routine are key, I think. Sometimes, you just have to force the motivation, but then you gain momentum.