I agree it seems odd that people who were able to live together in relative contentment for X years all of a sudden end up in divorce court at such a late date. My mother, soon to be married 40 years and counting, informs me that there seems to be a phenomenon sometimes called the “seven year itch.” There seems to be loosely/approximately seven-years-long stages in marriages, and you as a couple and as individuals must master the developmental issues particular to each stage in order to move on to the next. If you fail to master successfully the issues of each stage, then even 21-, 28-, 35-years old marriages can fall apart.
For example, the couple experiences an initial adjustment stage in the newlywed period, adjusting to each other, in-laws, etc. then you enter the early parenthood stage with young children, then older parenthood with teens, then the going-away-to-college and empty nest years, then all-day togetherness of retirement years. There are new issues requiring introspection, growth, communication, negotiation, and compromise all over again at each stage.
At the same time you are going through psychological/emotional developmental stages as individuals: career, parenthood, individual identity issues that may result in mid-life crises of asking “Who am I?” and “What do I have to show for my life?” and “Is it too late to change directions?” and going back to school, changing careers, etc. Evidently conflicts can arise from any of these issues that can tear apart even already long-lived marriages.
And then there’s the advice I’ve heard about “There are women out there who don’t care that you have a happy marriage and family, they will fixate on your husband, and do anything physical and emotional seduction-wise to get him for their own.” So never take your spouse for granted, continue paying attention to your own appearance and interesting conversation, continue to compliment and build up your spouse, etc. etc. so the “other pastures” won’t look greener by comparison.