When you wake up from a bad dream, you feel relief; but this relief at finding that the dream was only a dream soon passes, and you're left with the shadow of what was bad in it. 'Oh, it was so horrible: there I was, stranded - and, all around, those scowling treevles.' If, on the other hand, you wake up from a wonderful dream, your primary feeling is disappointment. Oh no, it wasn't real; beautiful but snatched away. The disappointment outweighs the good memory of what it was you thought you had.
I agree with what he says about bad dreams, but not quite on the good ones. True, if what made a dream good was simply the state of affairs it portrayed, then realisation of its falsity will burst the bubble. But what about cases where what’s good is largely the quality of the experience?
Two examples, from my own brain (not rude):
One of my cats died a few years ago, which was very upsetting. Some months later, I had a dream with her in it (not in a miraculous back-from-the-dead way but mundanely, as if it she’d not died at all). It was utterly realistic, down to the details of the markings on her face, where on her back she liked being stroked, and so forth. On waking, I felt a little rueful and melancholy, but also very warmly, fondly pleased. It was almost like having had a last chance to see her, and I was glad for that, in a funny sort of way.
In a completely different vein, I occasionally dream about flying (à la Superman, not easyJet). To be frank, I’m not all that good at it – there’s a fair amount of arm-flapping, and my landings lack a certain grace – but it’s good, exhilarating fun. When I wake up, it’s hard to be seriously disappointed that I can’t really fly, because that’s such an obvious fact. What I get to keep, though, is the memory of the sensation of flying. Sure, I know full well it’s a phoney memory, but I still like having it.