I get that same feeling. It definitely seems to be triggered by some daily stressors- but I feel so guilty for being depressed since I'm so fortunate. Part of it is my personality- I always strive to be at my best and I get frustrated when I don't meet overly high expectations. Part of me thinks that if I could adjust my outlook to just let myself be content- it would be easier but depression has a physical toll that it takes as well.