It's hard to imagine being in a relationship as a Hollywood actor for many reasons. They're constantly being scrutinized in the public eye, they're always surrounded by other talented and beautiful actors, and they're inevitably going to have their careers compared to their partners'. That last reason could lead to a couple splitting up, Scarlett Johansson warns.In Cosmopolitan's May issue, cover girl Johansson addresses that very real problem of constant competition. Although she avoids naming anyone, it seems that she could be referring to her previous divorce from actor Ryan Reynolds."The logistics of being with another actor are challenging," she told the magazine E! reports. "There has to be a real understanding of how you share your time, especially when two people's careers are going at the same rate. Or even if one person is more successful than the other, that also proves challenging. There may be a competitive thing."It's nearly impossible to predict whose working career is going to take off, and when, especially in an industry as fickle as Hollywood. After three years of marriage, Johansson and Reynolds divorced in 2011. Luckily, both remarried, and both even had super hero blockbusters — Reynolds in Deadpool and Johansson in Captain America: Civil War — both out this year.Let's hope they aren't comparing ticket sales.