Why does everyone hate Titanic?
Well, it's one of the top-grossing films of all time, so I'd say your question is based upon a false premise. A lot of people (as in, tens of millions) love or at least enjoy that movie.
Why do they like it?
1) Great production values; visually splendid.
2) Appealing actors, and romance.
3) Eye candy for history buffs and the many people fascinated by that ship.
I'm not that fond of it, myself, but I don't hate it. There are good and bad points.
It really does look great, and I love the scene where
Titanic leaves port at Southampton.
As actors, I like Winslet, DiCaprio and Billy Zane.
Kate Winslet gets nekkid, God bless 'er.
Now, I think we can stipulate that the story is not drama, but melodrama. There's nothing wrong with that; but my problem is that it's dull melodrama. For this, no one's to blame but James Cameron, because it's his script.
Cameron isn't a great writer. He's great at directing coherent, kinetic action scenes (hacks like Michael Bay could learn from him). His dialogue is fine for something like
Terminator. But he wasn't qualified to do a good melodrama set in 1912. (And he's not great at original ideas; but his cheerful appropriation of storylines is a whole other subject.)
Cameron really should have turned the script over to another writer; there are writers who eat, drink and breathe that period of history, and -- even keeping the hackneyed poor boy/unhappy rich girl romance -- could have produced a script with greater depth of character, vibrant dialogue, and layers of historical resonance. (Also better dramatic pacing; Cameron just does not know when to stop with the action sequences. Not only are our pair of lovers trying to survive on a sinking ship, but they're being shot at. Ridiculous. And it really annoyed me that, after Rose takes heroic action to get to Jack and set him loose, he immediately takes charge as if she's witless. He informs her that the passageway in which they're waist-deep in seawater is flooding. Then for five minutes the dialogue consists of him telling her "This way!" as he drags her through the ship.)
In short: disappointing script.
Also, there's that awful song.
And, according to a book I just read (
Titanic's Last Secrets, by Brad Matsen) it looks like the scene of the ship is sinking may be inaccurate. (I mean, the ship definitely did sink, but the details of exactly how that happened have been in dispute for many years, with some compelling new evidence having come to light recently.) Though we can't blame Cameron for that; he was working with the best information available at the time (1997).
Speaking of accuracy -- although the ship's band (or half of it; there were two bands that sometimes joined together) almost certainly did not play "Nearer My God to Thee", I'll give Cameron that one, because I love that scene.