Being with someone with similar personal and behavioural values you posses is the ultimate peace of mind. This is something love cannot change or help. Mostly values developed by a mixture how the were brought in the home and enviroment.

One person will surely keep complaining while the other keep trying to "hold themself back" because they're just falling in line to please the other person but this is not going to be sustainable because it will keep going for forever and the complain will never end. 

Surely they can learn and change over time but if they're a certain way after > 20 years. Are you willing to complain for >20 years with the hope that in that time they developed different values that will eventually suit the relationship? Definitely not