It's not hard to imagine why Cameron made this decision—thanks to Avatar, critics are crowing about 3-D as the future of cinema—but it is hard to think of a worse movie to be rendered into the third-dimension.
Cameron said at Comic Con that the reworked 3-D portions of Titanic he'd previewed looked "spectacular." Which may very well have been true, depending on what part of the movie he saw. There are shots that would lend themselves quite well to 3-D—the steam-room inside the belly of the ship, far-off views of it being loaded before it sets off, any depictions of churning arctic waters.
But none of these are the stuff that carries the movie. At its core, Titanic is a love story, not an action movie, and turning it 3-D would mock any sort of earnestness that may have been attached to it. Perhaps with 3D glasses on, the ship cracks in half with more gusto than before, but what about when Rose lets go of his hand and Jack's lifeless body slips into the ocean? The bodice-ripping romantic scenes and stilted phrases ("I'm King of the world!") in the original version of Titanic already push it deep into the embarrassment zone, and 3-D will make it much, much worse.