From the gossipy stuff I've read, their main issue was differing views of stardom. She has always embraced and enjoyed the celebrity lifestyle, viewing acting as a means toward increased visibility....while he views acting as a job that allows him to earn the money to do the stuff he really wants to do, like home improvement projects, video games, and traveling. She views being a celebrity as 24/7 kind of thing, and he likely felt he was being "dragged along" to events and parties he didn't want to attend, but only did so because she convinced him this stuff was part of being rich and famous. There is also some ink being spilled over the fact that Joe M is a recovering alcoholic and didn't want to be part of the Hollywood "scene" to protect his sobriety.
It reads a bit like the old 1960s sitcom Green Acres, where a small-town farmer marries a frothy New York socialite, only in Green Acres it was the wife who ended up down on the farm, whereas Joe M ended up being dragged into frothy, rich society life by Sofia V.