It takes both people to want a marriage to work, and both need to work at it to make it happen. I see too many people who want everything to be easy, or want their marriage to be heady love and romance all the time--and when that pink haze of emotional "love" finally wears off, then they are left to deal with the real person and all their problems--problems that had always been there but ignored or not addressed and/or worked on.
My husband and I both come from families where people enjoy long, long marriages, where people don't have affairs, where, if they're not "in love," they love each other enough to stay committed and work through issues.
We may not always agree or like what the other is doing or saying, but we talk things through and we work out problems and issues. I think a lot of people want to have everything, including the new heifer or bull on the other side of the fence. The problem is, that bull or heifer might not be all that you think it should be--should being the important word.
My hubby and I married each other because we loved each other, we believed in marriage til death do us part, we believed in working on something, of crafting a life together, of raising our family together. We weren't interested in having two different households with x number of relatives/fighting over whose side gets the kid for which holiday. We also made sure we had enough interests in common--that our principles and ways we look at things were in synch--and that the way we grew up was also compatible.
I'm not saying our marriage is easy or that life's been easy, but neither of us would ever, ever think of cheating or having an affair. We made a pact a long time ago that if the heifer or bull on the other side of the fence looked better than what we had at home, then a divorce was in order before that heifer or bull was ever pursued.
Neither of us has any gumption or need to chance after someone else. We have more than we can handle pursuing our shared goals and raising our daughter.