Our sources (not surprisingly) have two views of what happened ... some align with him and some with her.
The Jen sources say Ben's drinking and gambling have escalated over the last few years to the point "she couldn't take it anymore." Ben's sources also say he likes to drink and loves to gamble ... in fact, Ben has rhapsodized about gambling in interviews.
Sources say "partying" had nothing to do with their problems. Ben did not go out and did not womanize ... Jen complained he just drank and gambled too much.
As for our Ben sources, they say it's really just about 2 people who want to live their lives differently. His people say they've been in therapy for more than 2 years -- he even mentioned his marital problems during his 2013 Oscar acceptance speech. They add the couple has been separated for 10 months, so there was no recent "breaking point."
Ben's gambling is well-documented. TMZ got photos of the actor last September during a high-stakes blackjack session in Detroit. And TMZ broke the story, Ben was banned from the Hard Rock in Vegas in late April 2014 for counting cards.
The other thing our Ben sources say ... he wins big in gambling.