A psychological phenomenon that people with depression have a more accurate view of reality.

In general, depressed people have a more realistic perception of their importance, reputation, locus of control, and abilities. People without depression are more likely to have inflated self-images and look at the world through rose-colored glasses, thanks to cognitive dissonance and a variety of other defense mechanisms.

Of course incredibly depressed individuals can be unrealistically negative. Also note that this trend does not necessarily entail that a happy person is definitely delusional.

See also:
Cummins, R. A., & Nistico, H. (2002). Maintaining life satisfaction: The role of positive cognitive bias. Journal of Happiness Studies 3, 37-69.
Don't worry, Be happy.
total perspective vortex
Dobson, K. & Franche, R. L. (1989). A conceptual and empirical review of the depressive realism hypothesis. Canadian Journal of Behavioural Science 21, 419-433.