Most understandings are simplistic at best. Medication only manage the problem, they are not a cure. There are a myriad of things that could go wrong at any time, including building up an intolerance to the medications, or side effects that could prevent you from continuing the medication, or the sense that you feel so "well" you don't need medication any longer and stop taking it.
I've been through this with a family member for over 30 years - battling depression and anxiety disorders, etc. is like holding a beach ball under water; you have to constantly stay on top of it, otherwise, it pops out from under you and you sink right to the bottom.
It's to Greinke's credit and his doctors, that he's been well for this long and there's nothing to suggest that he's at risk of a relapse, but the underlying cause still exists and could still be a concern.
I'm sorry about your family member but I don't think that Greinke or Joey Votto fall into the same category. The pressure to succeed at the MLB level is far greater than what we mere mortals face.
