There is a lot of talk about the seven year itch. That is, losing interest in the existing partner, and developing interests in having sex with other partner outside the relationship, which occurs about seven years into the relationship.
Research shows that most people actually develop more of a four year itch. That is, after that initial stage of extreme romance, and passion, and desire, wears off, which usually ends after 36 months, that's when most people begin feeling interested in other partners. That is, it's usually the four year itch, the four years that mark the end of that initial lust, and limerance and passion, and that's when relationship becomes more burdensome, and passion requires work, and commitment.
It is usually after about seven years, that many people divorce, however, the process of that kind of relationship break down usually begins at about four year demarcation.
This four year phenomenon has been shown across many different cultures, and evolutionary psychologists explain it in evolutionary terms. That is, the most of, for our ancestors, four years was the time that it took for an offspring to become somewhat independent, and being able to function, without the presence of both parents.
The way that nature designed is, is that the four years, the initial four years, are marked by high dopamine, high adrenalin, those kind of butterflies in the stomach, a lot of desire, a lot of sex. After that time, a relationship becomes more mundane, and monogamy often requires effort, so that it doesn't turn into monotony.