Mental health, just like physical health is not absolute. What this means is that just like we're never 100% physically healthy, we're never 100% mentally healthy either. Mental health is never 100% or 0%. 

It's not an on and off switch. In fact, it's more like a pendulum. We have our good and bad phases.

A pendulum swings from one side to another. And at times, it may swing to one extreme. That is when we might need professional help to return to the safe zone.