What is his secret?

He sold his soul to Satan promising to make the best movie ever. In exchange, Satan would make sure it will be the best selling movie ever.

Then James proceeds to make a (secretly) terrible movie instead, but billed it as the best movie ever. Not knowing about modern movies, Satan made it the best selling movie ever as written in the contract.

Few years has passed, realizing that the movie was obviously not the best ever, Satan got a buyer remorse. James quickly offer to buy back his soul, for a discount. In return, James promised Satan he would make the best movie ever and offer his soul for Satan to collect when the time comes.

A few more years, James boasted that he would make the best movie ever (again), but for real this time. James sold his soul (again) to Satan with the same contractual agreements as they were in the last transaction.

James turned out made a (secretly) terrible movie (again), and billed it as the best movie ever (again.) Satan thus made the movie the best selling movie (again) not realizing that he's been tricked twice!

Soon enough, James came to offer to buy back his soul (again.) Of course, for a even bigger discount. Satan was furious, but ultimately backed down. James got his soul back. Satan swore to God that he will not be fooled again.

Everything seems to be over. Now, without Satan's help we won't see another best selling movie ever made by James anytime soon. But lucky for him, he got to keep his soul.

He understands what popular entertainment is - trash. Say what you will about him, but the Titanic is perfectly executed glossy trash. Then you've got The Terminator - fun trashy scifi pulp that doesn't make any sense and never tried to be anything other than a blockbuster. Hopefully Avatar 2 and 3 tank with his hippie hug the trees message, but he's such a household name that he's gonna have us all licking his balls.

If you think Titanic is terrible you are truly clueless. I guess you never grew out of the "hahah Titanic sucks Leonardo DeCraprio hahaha" phase.

Timing. He knows when to put a movie out.

Unfortunately, even though I consider both Titanic and Avatar as challenging and outstanding cinematic achievements, I have to agree with op.

Titanic had great visuals, decent acting, fairly effective drama and amazing tension... It was also too long, focused WAY TOO MUCH on the romance (to a point that I felt it was almost a movie made for teenage girls), wasn't very historically accurate and was totally overhyped. Yes, I give him that, he did a great job. It wasn't a "terrible" movie. But I personally don't care much for it and have no pleasure to watch the first cheesy and unnecessarily never-ending first half of the movie. Then the second half is definitely better, they still focus a bit too much on the romance and the family drama rather than the tragedy itself. So, yeah. I agree that it deserved pretty every technical oscars, but best picture? Good Will Hunting was a way better movie storywise that year.

And well then there is Avatar... How can I describe it: boring. Honestly, take out the visuals, and you get a 3 hour movie about how much humanity is a nuisance, how much humanity could learn if another specy from another planet could communicate and teach us and how, in the end, humans manage to destroy everything. Yeah, cool. I got it. It wasn't the first time it was shown and it certainly didn't need to be so long. The plot itself looks way bigger and more impressive than it actually is. The movie is basically a rip-off of Dances with the Wolves and Pocahontas with blue aliens istead of first nations. It was indeed visually impressive, and I salute the effort. But did I enjoy it? Hell no. I won't rewatch it anytime soon.

So yeah, his two most successful movies are in fact his weakest. However, I can see how they could appeal better to the mass audience because, well, they have the elements needed in a blockbuster.

Cameron simply understood how to make bankable movies and he will rather use this knowledge to make money instead of working on their plots.

Masonry

Between the two movies: 14 Oscars, 23 total nominations, both nominated for best Picture including a win for Titanic, 6 Golden Globes, 12 total nominations including best picture wins for both films, an 82% and 89% on Rotten Tomatoes, and both a 7.8 on IMDb. You're in the minority here.

The secret is that they are actually great movies