Hi RossMan
The issue of marriage and subsequent separation from marital unions often occurs for much of the same reasons that people go out to discover their "true calling" or have a midl-life crisis or some other sort of dramatic event.
These reasons are complex in number and even more complex in some of the theories driving behavior and outcome but I think it can be summed up with one word: change.
A huge part of life involves knowing what change is, anticipating change, and working to ensure that transitions are smooth. If you and your wife have a change and say have a child or move or experience turbulence, these are areas where change occurs and the response to this change determines whether your bond grows or has fissures and hidden places that are never visited. Assuming you built on a foundation of trust and openness, dealing with changes in the physical, emotional, and spiritual aspects of your partner's life and learning to genuinely care, and to fully love, growing in love, make the huge differences in long-term successes and failures.
Years pass by quickly. Couples find themselves at some point with kids, with jobs, and then start thinking to the dreams of youth to realize they are better off by themselves because through whatever means, they have a less-than-perfect relationship and having settled into this early on with poor choices, they do not see a way out except for divorce.
Also, many people do not understand what it means to love, and what it means to share a life with another human being. With so many things that can go wrong, one of them usually does wear down a marriage and make it one of conventience of mutual resentment or some other sort of arrangement.
If you want to feel the same way later, talk about it, and be responsive to change. Make the effort, and make your partner feel loved. It really is the little things in everyday life that make the difference in the long-term.
Well, that turned out to be really long. There's also no real formula. Love and caring umbrella the whole thing and every couple have a unique shared bond and shared memories that build, grow, and change. Working through the good and the bad is a matter left up to the individuals so since I doubt I know your wife as well as you do, you tell me what will make your marriage last- you're the expert here.
Cheers !