I'm a bit late to this party, but I'd say that I must emphasize that love is always a choice. You cannot fall out of love unless one or both of you has chosen to. You must work at it every single day, you must change, you must push yourself into behavior which might feel uncomfortable at first.

This is the entire basis upon which a marriage is built in the long-run. If physical attraction, humor, excitement, or anything like that is your principal and only motivation for a relationship and you (or they) have no inclinations beyond those, it is doomed to fail. You must make choices every day to love them and take their feelings into account. You must change behavior, you must stretch and you must frequently be ready to admit fault and try again. You must be brutal honest with them and with yourself and keep pushing. So long as they're willing to keep trying, there is no excuse for you to give up and leave.

For every situation in which you find yourself ruminating over how terrible their behavior is and you're certain they have not given up, as hard as it is to understand at the moment, you are likely the asshole. You have either not been honest about how you feel and YOU are actually the one treating them poorly because you're annoyed at something which you have never discussed with them, or you are utterly oblivious to their feelings.

I'd add on the end, when you come to an understanding of something you should apologize for but withhold it because you are attempting to extract an apology or understanding from them, you are in the wrong. Full stop. You are making a choice to withhold solace, admit fault, and comfort a loved one because you're afraid of making yourself seem or feel weak. You are emotionally manipulating them in a dishonest way. It doesn't matter if that means you're always the first one to apologize. You must apologize, or you reinforce behavior within yourself that your emotional needs supersede theirs and that it is ok for you to intentionally hurt them if it achieves a goal of yours.