When you are focusing too much on their faults and not remembering their strengths, and wanting to change them too much. Often we get so involved in this critical mind, that we forget their strengths and when they manifest their human weaknesses we are unable to have compassion for them and forget their strengths.
Learning how to appreciate your partner more by reflecting that they may be dead tomorrow is very helpful. To break through this ultra critical mind that doesn't appreciate who they are now. When we are always trying to change them into something they are not and being ultra critical is when we are taking them for granted. When we are lacking in generosity, sometimes, when we are expecting too much from them, we are taking them for granted.