Mine just had an aversion to the old "ball and chain scenario". Marriage in general made him ill. He stopped wearing the wedding ring and did not want any responsibility related to family. He even said to me while married, "the dating scene out there is horrible", WTF......
Snodderly's advice is excellent. Protecting the marital assets at this point is key. My ex was on a wild wild spending spree, spending $50,000 - 70,000 per month on himself. I filed for divorce to save the assets. He did not care what he did.
Me: 46 H:44 Together: 25 years Married: 20 years Separated: 11-30-06 Divorced 12-21-07 OW: EA began 2005 PA began end of 2006 3 children,20, 16, 6 ex asked for forgiveness 01/16/11