How many times have I heard that "our relationship isn't what it used to be when we first met"? "We don't have that passion any longer."
Those days, weeks and possibly months, are or were filled with passion, longing, and unbelievable full-throttle attraction. And then?
What is happening is that your brain is "lighting up" much like being on a hard-core drug. And that feel-good chemical in the brain is known as dopamine.
Romantic relationships then begin to mature.