You know it's not always a reason, or an intellectual process. In my case you could say it was 'existential incompatibility' - I still believed in everything but just couldn't do it anymore.
But if you really don't care then it wouldn't even occur to you what you don't care about would it? You know it's one thing if you ask things like do you still care about the lack of love or whatever, but if it didn't cross the person's mind on it's own then that tells you something.