The Long Haul?
What about when that euphoric feeling is gone? According to Ted Huston at the University of Texas, the speed at which courtship progresses often determines the ultimate success of the relationship. What they found was that the longer the courtship, the stronger the long-term relationship.
The feelings of passionate love, however, do lose their strength over time. Studies have shown that passionate love fades quickly and is nearly gone after two or three years. The chemicals responsible for "that lovin' feeling" (adrenaline, dopamine, norepinephrine, phenylethylamine, etc.) dwindle. Suddenly your lover has faults. Why has he or she changed, you may wonder. Actually, your partner probably hasn't changed at all; it's just that you're now able to see him or her rationally, rather than through the blinding hormones of infatuation and passionate love. At this stage, the relationship is either strong enough to endure, or the relationship ends.
One study examined the fading of love through the lens of "self expansion," the concept that people have a fundamental motive to expand their sense of self through the formation of close relationships. While new romantic partners tend to continuously learn new things about each other early on in their relationships (which generates passion for one another), over time, "self-expansion opportunities diminish and less passion is generated." Researchers concluded that people in long-term relationships reported lower levels of self-expansion in their relationships than people who had been with partners for shorter periods of time and "while self-expansion is not a panacea that will cure all relationship ills, it is one of the more promising keys to enhancing positive affect between long-term partners who seem to have grown bored."
If the relationship can advance, then other chemicals kick in. Endorphins, for example, are still providing a sense of well-being and security. Additionally, oxytocin is still released when you're having sex, producing feelings of satisfaction and attachment. Vasopressin also continues to play a role in attachment.