I agree, it's all about larger issues, but when one spouse or the other will not address the underlying issue, you really have to apply those DB principles.
One spouse chooses to 'run away' from it with the thinking that if I ignore it, do not address it, it will go away and at a minimum-be a non-issue for me.
While (as 17B says) that in turn, causes an almost knee-jerk reaction in the other spouse to hold on and cling. Which of course, as we all know, does not work.
I like to call that one escapism. I sometimes think my H compares to Houdini! I'm not sure if I would classify it as apathy exactly, but plain and simple ol'running out.
Sure would like to understand what/why that need is felt, MLC/disinterest in marriage, or maybe simply because it's easier(?)...NOT knowing reasons for this discontent tends to 'force' the left-behind spouse to look to themselves to blame. At least in my case-when there is no reason given, it MUST be because of me, right? Rationally I know that is not true. However, my emotional-self struggles.