I just posted an answer to a very similar question, so I’ll copy it here:
New love triggers massive releases of the neurotransmitter dopamine, which feels fantastic. It’s a major component of what we mean when we talk about the feeling of being in love. And it is literally addictive (dopamine addiction is at the heart of gambling addiction, for instance).
But this heavy dopamine release doesn’t last forever. Eventually, being in the presence of that “special person” no longer gives you that dopamine high. This has some evolutionary benefit, because while riding that high is great for procreation, it makes one rather dysfunctional in other aspects of life.
The effect is that we can go through a kind of dopamine withdrawal, where we start looking for a new way to generate that dopamine release.
There is another neurotransmitter, oxytocin, which is associated more with the pleasure of companionship— a “cuddlier”, less passionate feeling than that delivered by dopamine. oxytocin is much lower-key than dopamine, and doesn’t have the same addictive quality. The success of long-term relationships seems to depend on being able to be content with the glow of oxytocin rather than the blaze of dopamine.