Everyone's got their theories on the curse- what it's for, why Rumpelstiltskin made it, and so forth. This is my own theory, in drabble form.
Forever is such a nasty word. It trickles down his skin like grains of sand, counting off the years that have vanished without a trace. Forever is too long to live in guilt and self-loathing. Too long to watch his heart break over and over as the ones he loves are taken away.
This is damnation, the price he must pay for a power he didn't understand.
He's committed crimes—no doubt about that. Crimes without measure, though not without number—oh, he's kept a careful ledger, calculated the weeks and months and years that any reasonable man will rot in the belly of hell. He's measured that time against the strength of his own sanity, and come with a final, happy number. A compromise.
Twenty-eight.
Twenty-eight years he will spend living out the same day over and over again, caged within his own guilt, in a prison of his own making. Any longer than that and he might lose his mind. Any longer than that and the hero-child will have found peace and family of her own in another land, and curiosity about her homeland will wither and die.
No, it must be twenty-eight.
He aggravates the process, of course: lames himself with bits of chipped cup, traps himself in the guise that Bae last knew him in. That's the face he'll have to look in the mirror for twenty-eight years, and he'll wear it as gracefully as only a convict can. After a decade or so, he'll even stop flinching at his reflection.
Maybe, after all that, he'll die. Maybe he'll be thrown into another cell in another world. Maybe he'll forgive himself and try, one last time, to be happy.
Because that's what this is all about, isn't it?
And when he looks around at the witches and ogres and dragons, at the peasants who don't get to marry princes and the virgins who don't get saved from their sacrificial altars—
He pities them. Almost as much as he loathes himself.
So he scoops them up, arranges them in a dollhouse of his own design, dresses them up with new names and new memories. He still hates them, envies them for those little chances they do have—the ones he'll never have again—but maybe it'll be different in another life.
Maybe things will change.
And so he weaves the curse, the blessing, the baptism through time and war, and sets its price: twenty-eight years from every soul.
And twenty-eight more from the man with no soul left to give.