Okay, I'll try to address some of the questions posed here, although they may be rhetorical...Doug and Kayleen are walking wounded, human train wrecks who desperately need to heal each other and don't know how. If they felt they were worthy of love and could accept it, then they wouldn't even be in the positions they're in. The miracle would be for them to be able to do that, and Joseph ends the play with the hope that finally, they just might. Their backstories are left vague, but there's no doubt from the pain they've inflicted on themselves over the years, that their own families of origin would have had to be deeply dysfunctional.
Now whether or not that makes moving, illuminating theater is a personal thing; but psychologically, the characters seem to make perfect sense to me--if you figure that they've both been pretty much damaged goods from childhood on. I was touched by their pain and their plight, but that doesn't mean you have to be.