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It goes because the "spark" is nothing more than hormones going off in your head. Hormones that are triggered by a combination of excitement, unfamiliarity and base attraction. These hormones are designed to aid in procreation - saying "this one! mate with this one!".
But once the unfamiliarity and excitement wear off, and the mystery gone, the spark leaves also.
The "spark" has about as much to do with love as alligators have to do with microchips, and confusion between the two is the root cause of the majority of failed relationships.