The person who wants sex more often may start thinking, “Why am I always being rejected? Maybe they do not desire me anymore.” The partner who wants it less often may hear something entirely different: “Why is something constantly expected of me? Why do I have to keep explaining myself?”
Gradually, sex stops meaning only sex. It can become a measure of love, attractiveness, attention, obligation and even personal worth.
Desire is not a fixed setting. It changes with sleep, stress, health, age, children, work, emotional state, relationship quality and many other factors.
So two people having perfectly matched libidos for many years is probably more of a fortunate coincidence than a realistic standard.