You want to know why Scarlett and Sean broke up??

Sean Penn isn't looking to be tied down.

An insider tells the new issue of Us Weekly (on stands now) that Penn, 50, broke up with Scarlett Johansson, 25, after five months because he has not wanted anything "serious" since his 2010 divorce from Robin Wright, adding, "Scarlett pursued him."

On June 4, the Avengers star didn't seem ready to give up on their unlikely romance. At Spike TV's Guys Choice Awards in L.A., she scrambled for face time with the Oscar winner. "She pushed herself into his conversations," a witness tells Us. "She sat down between him and Robert De Niro. Robert was like 'What?' and made a face."

Johansson also made sure they were snapped together. "A photographer asked for a shot," the source says. "Before Sean could say no, she said yes."

Tell Us: Are you sad to see Scarlett and Sean split?