May be feeling checked out, or like ur drowning in each other already, or maybe he feels a lack of engagement from you and is reciprocating it. These things take a careful balance. Romance is a way to draw closer to someone; that means there has to be enough daylight to make that meaningful (not much, just have your own lives), and also enough fondness that he enjoys romancing you. When I’ve tapered off over time it was usually that last thing: she appreciated it less and I liked her less