Your depressed friend's good mood doesn't mean they're cured. So much of the world's population is depressed, you'd think we'd all have a clear understanding of it by now. One in four people in the UK are thought to struggle with their mental health.
But (stay with me here, I'm not being hyperbolic) depression isn't like a regular illness, like the flu, it's more of a complicated disease that we don't fully understand, can't yet totally cure, affects everyone differently and has different stages, and that can often be fatal. It's more like cancer. Just like cancer, the illness can be 'beaten' and bodies can be rid of it, but they'll need regular checks to monitor a possible return.
Just like cancer can happen to the healthiest of people and depression can happen to the funniest, a minor blip of strength is something that should be celebrated - of course - but doesn't mean that the patient is 'all better'.
Often the moments of strength are simply all a show for friends and family anyway, as we all tend to want to conceal our true pain from those we love.