Functional Depression

[bikes]

just because people can function doesn't mean they aren't depressed.

[KoRnholio]

just because people can function doesn't mean they aren't depressed.

Indeed.

Depression (and most mental illnesses) are not like most medical diseases where it's a simple matter of "diagnosis = yes or no". It's a matter of severity and how much of a detriment it is to one's lifestyle.
The definition of clinical level depression involves significant disruptions in functioning, eg work/sleep/personal life.

There are plenty of slightly and moderately depressed people out there who can get by without counselling or medication. But even if the depression isn't at a clinical level, one would often still benefit from counselling and perhaps even medication to improve their life/functioning.