The source says Johansson, in particular, was feeling the strain, and that Reynolds tried to improve their situation.
"At first it was ... let's fix this," the source says. "And then it got to the point where it was just, if it's not working, it's not working. He's an adult, and he finally just said they should stop."
The couple's age difference – she is 26, he is 34 – may also have been a factor. "She's young, and that played into how she's able to deal with things," the source says.
The source believes the couple decided "pretty recently" to divorce, before the problems got any worse. "They're doing it this way because there's no drama, there's no cheating, there's no nothing. These are just two people who love each other who are not going to be together anymore."
As for reports that Reynolds was involved with costar Blake Lively, several sources insist it isn't true. "They worked together and got along really well," says a source. "But there's not more to it."
Another source adds that Reynolds is planning to let Johansson file for divorce first. And following the dissolution of their two-year marriage, healing will take time.
"Ultimately, this is the best thing to keep both of them happy and healthy in the long run," the source says. "But yeah, he's sad. They both are."