Italian scientists have determined that the changes to body chemistry which causes people to find new partners sexually attractive lasts, at most, two years.
At that point, couples shift into a "stable relationship" period, and other hormones dominate.
Hormones called neutrophins were present in much greater quantities in the blood of those in the early stages of a relationship. Testosterone also increased in women and decreased in men during this period.
This changed in those who had been in a relationship between one and two years; the "lust molecule" neutrophins were replaced by oxytocin, the so-called "cuddle hormone".
Some researchers were concerned that people might feel obligated to take hormone supplements in order to re-experience the initial rush of lust.
As for the discovery of the neutrophin hormones themselves, I can't help but wonder if this is related to the fact that couples who become disillusioned and negative towards each other early on generally do not stay together; studies show that the couples most likely to be married for the long-term are those who maintain their positive feelings for their spouse for at least the first two years.
It sounds very much as if couples need to last through the initial stage of attraction characterized by neutrophins, and reach the "steady state" in which oxytocin takes over, if they are to last. Even couples who are particularly affectionate as newlyweds usually divorce if they do not make it past that crucial period.
Similarly, couples who have an extremely short courtship prior to marriage are at greater risk of separating. They may be marrying in the initial rush of attraction, before they know if they can make a go of it long-term. (Couples who put off marrying for a very long time are also more likely to divorce, possibly because they have often put off marrying due to real problems in the relationship that already exist.)
This isn't, of course, to say that the hormones are determining the course of the relationship; the hormones are likely produced when the mind experiences the positive feedback of a good relationship, and then they in turn help keep the relationship going. Yet another example of the fascinating connection between the body and the mind.