I just got home from officiating at a wedding for some friends of mine who are just now in their 60's.  Barely, but there.  I didn't expect it to be such an eye opener about relationships, which is all curled up with the "World Oldest Toddler Award" (thanks Anne Lamott), as I learn something I wish I had known 40 years ago.  Which is that a successful marriage demands much more than "love" and certainly more than awesome chemistry.  Shared beliefs and dreams and goals are the stuff that appear to make a relationship and a marriage work.
My first marriage was in 1970.  We were gifted a trip around the world for our honeymoon, and in a railway station in India, as I sat on the floor chilling with the beggars and my young husband ranted and raved at officials who were not being efficient, I knew we weren't going to make it.  Somewhere along in there, a year or so after our wedding, and 3 years after getting together, we had the conversation that we should have had long before, had we only known to have it.  It contained such pertinent topics as "What do you want life to be like?"  "Where do you want to live, city or country?"  "Do you want children?"  and some variation of "Do you believe in and/or have a relationship with God?"  As it turned out, what we each envisioned was almost the polar opposite of each other. He wanted to be rich and live in NYC, which he did, and I wanted to travel as long as possible, which I did.  We didn't do it together because we had 2 completely separate lives and visions and dreams going on.  But the kicker for me was this conversation, which I'll try to record accurately, these 40 some years later:
We were talking about a friend who had no money at all but was so free and happy.  I said it was because happiness comes from inside you.  He responded that was nonsense, that happiness comes from what you have.  In that instant, I knew it was the end of the conversation, of the marriage, and of the relationship.  Did I still "love" him?  I put it in quotes cause it is so overused and little understood, but yes, I did.   I just didn't want to live my life with a man who thought the way to be happy was to have more stuff.  
On to marriage #2, which included 2 children.  We liked many of the same things:  he was an intrepid traveler, like me, and we had great adventures together, even some really scary ones.   We both loved great food and music.  He loved sailing, I didn't, but we were doing all right until I met my spiritual teacher.  When I told him, he looked at me like I had fallen out of a space shipand inside my head I heard the Voice say, oh boy, here we go again.
So for me, the most important agreement is usually about spiritual life, a relationship to the Divine, and how that translates into daily life.  There are also agreements about food (very important) and music (very important) and hygiene (very important) that can make things easy and groovy or difficult and contentious, but the deal breaker has, for me, usually been about God.
I had this thought today, driving home from the wedding, that a good happy marriage probably contains shared beliefs.  They might be, "We'll never have the money to travel,"  or "People like us don't go to college," or any variation.  If both people in the marriage agree, there is potential to be happy together, living in an agreement.  My friends who just got married happen to share the belief that anything is possible, and so their life together is full of joy and creativity and fulfillment.  Their agreement is to create the most beautiful life they can and share it generously with the people in their life.  They do.