Justin Theroux is reportedly a much more serious character than Jennifer Aniston.

The loved up couple have become one of Hollywood's most powerful pairs, with media outlets constantly speculating about when they'll tie the knot.

Justin and Friends superstar Jennifer got engaged in January 2012, after getting together in May 2011.

Body experts recently analyzed their behavior towards one another at the premiere of Justin's new TV show The Leftovers, with 45-year-old Jennifer jolly and playful and Justin, 42, more rigid, but friends say this is simply reflective of their character traits.

"That's the message we've been getting for some time now. Justin is a much more serious character than Jen, and while he's crazy about her, it's proving difficult for him to change his ways. He's worried about committing to some dream existence that he doesn't believe in," an insider revealed to British magazine Grazia.

The insider added that while Justin wants to make an honest woman out of Jennifer, who divorced Brad Pitt in 2005, he's unsure if he'll be able to make her happy.

Part of the problem is his apparent aversion to public spectacles, which is why he's not often papped out and about.

"In theory, he would love nothing better than to marry Jennifer. In practice, he worries that if they go for the big white wedding, with all the pressure that entails, they're going to jinx what they've got.

"It's no coincidence that he was with his last girlfriend, Heidi Bivens, for a decade without any talk of marriage. He's just not into all the pomp and ceremony," the source included.