Healthy marriages have healthy boundaries
Healthy marriages are characterized by healthy boundaries. A boundary is something that separates one thing from another. When two people are in an intimate relationship (like a marriage) we can think about that relationship as being bounded. The two relationship partners share secrets and experiences with one another that are not shared with other people as though there is a literal boundary or barrier that keeps these secrets and experiences within their shared private space.
The boundary around a healthy marriage is a flexible thing; it needs to be able to bend but it should never break. Although there may be strain that develops within a marriage, a healthy couple ultimately continues to act as a unit (or at least to act in concert with one another's desires) despite the best efforts of the world and others around them to pull them in different directions. For example, a healthy couple doesn't allow parents who are critical of their relationship to break that relationship in two, nor will they allow their child to play them against each other. A healthy couple will not break confidences or promises they have made with and to each other. Maintaining the boundary around the marriage means making the health of the marriage the first priority, even in the face of other 'first priority' activities such as parenting.
At the same time that healthy married partners keep their marriage as their number one priority, they are also not enmeshed; not joined at the hip. Each partner participates in relationships outside the marriage (family, friends, employment, etc.) and allows themselves to be influenced by those other relationships. The healthy marriage boundary can stretch to accommodate this activity. However, if push comes to shove, healthy married partners close ranks and act as a unit independent of outsiders (In-laws and even children are considered outsiders in this context!).
Healthy marriage partners act positively towards each other
Marital satisfaction is affected by how frequently partners get into conflicts, but not by whether they get into conflicts at all. Marriages vary widely in terms of how much conflict the partners tolerate. Partners in a volatile marriage are highly expressive and willing to give and take a fairly large amount of conflict, whereas partners in a conflict-avoiding marriage, by definition, try to minimize clashes and downplay displays of emotions. What distinguishes these two groups the most is the intensity with which partners attempt to change their partner's minds. These differences in willingness to bicker and fight appear to be normal variations in how partners communicate and are not particularly significant in themselves. It is only when bickering and fighting between spouses results in lasting hatred or hurt feelings that it suggests anything about the health of the relationship.
If the extent to which partners are willing to have conflict with one another doesn't tell you much about the health of their relationship, the relative amount of time they spend in conflict with one another vs. having more positive interactions does. Healthy stable couples are observed to produce about five positive (happy, pleasant) situations for each negative (angry, hostile, upset) one. Couples whose marriages are in trouble are substantially less positive towards each other than couples with healthier marriages. These findings suggest that it is not how willing one partner is to attack the other that indicates problems within the marriage; it is the frequency of those attacking episodes that is associated with marital problems.
Marriages vary with regard to closeness over time
Marital satisfaction is never completely constant, even in healthy marriages. All marriages tend to be experienced as becoming less satisfying as time passes. As one would expect, marital happiness and satisfaction are highest during the first several 'honeymoon' years of togetherness, and tend to drop to lower but still satisfying levels as time passes. The initial drop in marital satisfaction that is so commonly experienced appears to occur as each partner develops a more realistic appreciation of what they can expect and not expect from their spouse, and also as young children are introduced into the marriage. Major family changes and disruptions such as the birth of children and transitions of children into and out of school are particularly stressful times for most marriages, as reflected by partners' decreased satisfaction ratings.
The lowering of marital satisfaction level over time with mild changing levels of affection and attentiveness that spouses may have for each other appear to be a normal part of married life and nothing to overly worry about. Cause for concern only occurs when partners' periods of detachment extend for very long periods of time.