In order to best understand how a marriage can come apart, it is helpful first to understand some of the ways that healthy marriages are structured, and how they function.
Healthy marriage partners are compatible partners
In a marriage that is to stand the test of time, romance is important, but compatibility is critical. By and large, partners in healthy marriages come to agree upon common plans regarding the directions their marriage will take, and the way each partner will behave. These common agreements may never have been discussed, but they will be present in how each partner chooses to act.
Areas of agreement that partners will have dealt with will generally include:
Friendship. Successful partners develop a significant friendship at the core of their relationship. They genuinely like one another, amuse and comfort one another, and prefer to spend time with each other. This friendship and mutual liking is somewhat separate from other aspects of the relationship (sexuality, for instance), and can survive the loss of these other aspects of the relationship. A strong friendship and mutual liking is often the basis for repair of troubled relationships.
Role expectations. The partners reach agreement with regard to how household responsibilities are divided and how they will behave towards each other. Traditionally, and still dominantly, the male or masculine-identified partner will take on the majority of financial obligations, while the female or feminine-identified partner will take on nurturing roles. Tradition has broken down significantly in Western culture over the last century, however, and it is not at all uncommon to find 'women' who take on financial obligations, 'men' who take on nurturing roles, or to find both partners sharing these roles to one degree or another. Failure to reach agreement with regard to roles can be a major source of conflict.
Emotional intimacy. Successful partners learn to trust each other, to be vulnerable with each other, to laugh together, and to support one another in times of need.
Sexual expectations. Partners come to basic agreements as to how they will be sexual with each other. Frequently (traditionally) this means that they will be sexual with one another, and not with other people, but this is not necessarily the case. Sexual expectations may further dictate the kinds and patterns of sexual activities that each partner will and will not engage in. Coming to agreement with regard to sexuality can increase trust that couples feel for each other, and failure to reach agreement can be cause for conflict. As sexual activity is strongly rewarding and bonding for couples, it is best for marriages when partners agree upon sexual expectations and are both satisfied with their lovemaking.
Vision/Goals. Successful partners agree that they want to pursue the same life paths, values and goals and mutually commit to those paths, values and goals. Examples might include decisions to have children or not, to attend or not attend religious services, to raise a child in a particular faith, to save or spend money, or to live frugally or extravagantly, etc.
Successful marriages tend to be populated by partners who come to their marriage with pre-existing significant compatibilities (of personality, temperament, goals, etc.) that make it easier for them to reach agreement because they frequently end up wanting the same thing. They may share commonalities with regard to personality, temperament, or preferences for volatile or conflict-avoiding interactions, as well as goals, religious and ethical ideals, etc.
While these areas of agreement do tend to be present in healthy marriages, we should note that no marriage is perfect, and that many perfectly good marriages have disagreements with regard to some of the areas we've discussed. In general, however, the more areas you and your partner are in agreement on, the better are your chances for a healthy marriage.
Background factors play a minor role in determining marriage success.
Personality, temperament and goal compatibility is very important in determining whether a marriage will be strong. Other background factors are also important, however. Better marriages are reported by people who chose to marry later in life as opposed to younger, by people who remember being very intensely in love with their partners prior to getting married, and by people who maintain close family relationships and whose parents' approved of their marriage. Also, people identified with more traditional sex-role and religious values tend to report having higher quality marriages overall (although it isn't clear that such people aren't just reporting positive outcomes based on their desire to present themselves in a positive light). When all factors relating to marital adjustment are considered together, personality and life-goal compatibility seems to be of paramount importance, and background factors such as whether partners come from similar family, religious or economic backgrounds or whether they have similar dating histories appear to be of lessor importance.