Well, if your disorder isn't effecting your life in some way, then it doesn't really qualify for diagnosis. That's pretty much how psychology differentiates between an illness and harmless eccentricity. However, there are lots of ways people struggle that are less obvious than losing jobs or having no close relationships and that can make it hard even for professionals to determine what's going on.

I was always functional enough to "get by". I don't generally have the sort of extreme behaviors or outbursts that could get someone hospitalized. But the depression was still seriously screwing up my life, making it impossible to maintain jobs or relationships the way I should've been able to.