Alex Pettyfer & Dianna Agron Split: What Went Wrong?

Insiders have a theory as to why the gorgeous duo broke up

So much for the young couple we were all rooting for!

Now that Dianna Agron and her I Am Number Four costar Alex Pettyfer have split—a day after their movie hit theaters, and a week after funky engagement rumors—people are wondering what went wrong with the so in love couple.

Well...

A source close to the once cohabitating duo tells us that Alex was "a little controlling."

"He would always take her phone and read her messages and seemed really paranoid that she would cheat," our insider dishes. "[Dianna] would never do that."

Tabloids have speculated that Alex was in fact the one with a wandering...eye during their relationship, and our source played coy in answering that one.

"Let's just say a lot of us think his crazy suspicions came from somewhere. And it wasn't worry over what Dianna was doing."

Anyone who's been in a relationship where your significant other has cheated may take that paranoid type behavior to be a sign that they are doing something wrong.

But let's not be quick to judge just yet.

"They were very in love, they are young, sometimes things just don't work out," adds another source close to Alex.

Hearing all this, we're not too surprised engagement rumors hit the blogs before breakup ones.

They did have a movie to sell, after all, and two so-in-love leads breaking up wouldn't exactly do wonders for box-office numbers. Is that why the Number Four came in at No. 2?

Now you all know why Summit was so worried when Robsten first got together.