How about if the other person's behavior is something which you cannot ignore and of which you do not approve (nothing illegal or immoral, just foreign to YOU).
You discuss it and believe you have reached an understanding, but the person continues to do it - sneaking about in order to do so. The person makes repeated promises s/he does not keep, thus causing trust to die first and respect to die soon after that.
Marriage counseling was tried, but did nothing to change the source of the conflict between you.
No trust means no respect which means no love. End of the marriage, beginning of the divorce action. It is almost never simple, but there is almost always a reason.