I've been married for 34 years. My wife and me are good friends. We are close most of the time. I have been madly in love with her some years and some years just love. I have also been just content to stay with her when the feelings were getting dim.
For me it is about commitment and keeping your vows. It is not always easy and sometimes I don't feel like it's fun anymore but, after some time passes I remember the good things we have together like our 2 children and 6 Grandchildren.
I don't want to give up all of what I have just to satisfy my own selfishness. Besides that I don't want to start over again with someone else. Once this marriage is over through the death of either one of us, we know that we will not marry again. Till them I am happy to be with her and she with me.