While it is far form a universal rule, I think most people who see lots of separating or divorcing couples (Therapists, Accountants, Lawyers, Judges, etc.) would agree that while the underlying problems may be more subtle and hard to identify, there is usually a triggering event that pushes one or the other partner over the line from ambivalence to a decision to move on.
Over time, most households that are responsible with their money begin to accumulate long-term assets. These assets commonly fall into three categories; Real Estate, Regular Investments and Retirement Investments. If a couple is married and later decide to divorce, then a whole bunch of rules come into play, some created by courts as “common law” doctrines, others put in place by legislatures in the form of “statutes”, and yet others, in the form of IRS regulations and practices, governing tax treatment of gains and losses.
Census data makes it clear that fewer people are choosing legal marriage than they used to, or are putting it off until later in their lives. However, many people are living together, and gradually move into sharing their finances without ever crossing a clear legal line. Unless the unmarried partners have a written agreement governing their finances and what will happen if they break up, non-married joint finances are a train wreck waiting to happen. If Long Term Assets are involved, the wreck will likely be worse.