Sure, in the first stage of your relationship, sex happens spontaneously with an excess of passion and willingness. Even those who may have a hard time getting turned on find their desire levels off the chart as inhibitions fall away and you can't get enough of each other. But this fades as life returns to normal.
There are deadlines to make, family to visit, unplanned taxes to manage. Little irritations begin to crop up, power struggles close you down, and the responsibilities of life take over. You look at money differently, and one of you is an introvert while the other loves huge crowds … Among all that, and probably more, it's no wonder you ask yourself, "Where did the passion go?" as you long for that magical time again.