The angle would be that he's already depressed, in the medical not the colloquial sense (it's set after and so the end, when keller notes that he's not well and she's giving him sleeing pills) and he sort of sees Cam as the one good thing he has that's just his. And so it's less that he can't cope with losing Cam secifically than that he can't cope with one more thing and that happens to be the one more thing.

And yes, more retreating than cutting and suicide attempts!

Oddly, the very first idea I had for a cam/john fic (which I never wrote) was basically the flip of this-john's stranded on earth and they think atlantis is lost, and he's with cam and pretty deressed.