You may be right if you were only talking about the former. However, MDD, BPAD etc. are serious stuff.
Source: I'm a doctor and have seen serious depression.
You may be able to identify "major depression" but how many times is it just a case of "lack of meaningful purpose" and just "bad decisions" that are compensated for with drugs that may do more harm than good?
I don't even know where to begin answering this. There are criteria for MDD. Everyone's case is different and complex. There is always a judgement call to be made. There are tools such as the K10 that go a way to making an objective assessment but I actually find the difference between MDD and "feeling down" pretty obvious. Like most of medicine, it's pattern recognition.