Marriage is a complicated matter and there are many multi-layered issues on why they don't work (for bad marriages).
Of course there may be trust and commitment issues.
But I believe the biggest issue is that Person A is trying to change Person B (and vice versa) to be more compatible instead of accepting him/her as they are. Obviously, there would be no issues for Person A if Person B does everything he/she says. Person A also wants full control of Person B (or vice versa). So what happens is that one person gives in to keep peace, but he/she is slowly growing more resentful. Passive-aggressiveness and other foul behavior may grow during this time. Marriage involves a lot of compromise, and sometimes Person A may want Person B to make more compromises than he/she is willing.