Joseph:
Because I am witness to so many cases of dead marriages that cannot be resuscitated, after affairs, outright abuse of various forms, and multiple failures at reconciliation. The spouse proposing this has usually approached therapists in the past, and presented their case in the likes of, “My spouse is sick and I am healthy. Fix him/her, and then we will talk.”
The sincerity you imply is actually uncommon. I agree that any chance to restore a marriage should be considered a worthwhile investment. But the motive needs to be honest. In the vast majority of cases, it isn’t. I speak from extensive experience. You don’t need to believe an anonymous blogger. That’s why I suggested that one ask any other askan, dayan, to’en, and therapist who deals with this subject.
As for the unilateral demand for divorce, just ponder this. marriage requires two partners with a shared goal to be happy together. When someone doesn’t want to be married to you, you will find it impossible to be married to them. For additional support for this concept, check out the posuk in Mishlei – כמים הפנים לפנים כן לב האדם לאדם.