Wire Image / WENN 1 / 6
Wire Image / WENN 1 / 6

By Kat Giantis

What mysterious chemical reaction drew Scarlett Johansson and Sean Penn together? Was it their common interest in helping Haiti? A mutual disdain for the paparazzi? Or was it something a little more dangerous (and stinky), like a shared addiction to nicotine?

Their rumored romance kicked off with a sighting of them sucking on cigarettes outside a star-packed bash in Los Angeles, and a smoky haze apparently hung over their lobster- and margarita-fueled getaway to Mexico last week.

A spy tells Us Weekly (via HollywoodLife) that Scarlett, 26, and Sean, 50, were "kissing a lot and chain-smoking" during their overnight trip to Cabo San Lucas, noting, "They were acting like teenagers in love and were all over each other."

Seems despite Penn's crankypants reputation and bitterness over his divorce from Robin Wright (in January, he griped how he "got taken for one half of everything I had"), he "lifts [Scarlett's] spirits" and will "do anything she wants to make her happy."

Need a second for your stomach to stop churning? We'll wait … OK, brace yourself …

"She's head over heels," gushes a ScarJo insider, "[and] he feels like the luckiest man alive."

RELATED: Scarlett's feet seen near Sean's lap

Their mutual appreciation society met up this week at the Chateau Marmont in Los Angeles, where they hugged hello before heading to the restaurant, reports People.

The two ended up in a quiet corner table in the back and seemed "at ease" as they "talked, smiled and remained focused on each other."

On the menu: a salad (for her), a burger (for him) and wine, along with some brief across-the-table handholding.

"They looked genuinely happy together," says a spy.

When Scarlett and Sean made their exit, his arm found its way around her lower back. No word on whether he offered her a light.

RELATED: Ryan rebounds with German model

As for the basis of their attraction, Star (via Jezebel) has a theory: The not quite former Mrs. Ryan Reynolds likes the aura of worldliness that surrounds the onetime Mr. Madonna.

"She looks to Sean as an older, sexy, college-professor type," a source tells the tab. "She thinks he's a genius and she's totally into him."

Not that it's necessarily a romance for the ages: The two-time Oscar-winning actor supposedly told the starlet that he's done with the domestic scene, which may have gone over just fine, as she allegedly wants to "sow her wild oats."

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