"It's easier to maintain a fire than it is to reignite a spark!"
Oftentimes the problem is romance/passion is not a priority for (one) of the people. I've never heard of any couple waking up one morning saying together:
"Lets stop having sex, being romantic and passionate!"
Generally what happens is (one) them makes them self less available to the other by either rejecting them enough time or demonstrating a complete lack of interest in not too subtle ways.
Eventually the other person throws in the towel.
This is why it's nearly impossible for one person to "save a marriage". While it takes two in order to make a marriage work it only takes (one) to destroy it.
Communication is the GPS for relationships!
It allows you to know whether or not you're growing together or growing apart. The first thing you need to find out is if (they) are happy with the way things are. If they are happy it's not good.
People by and large only change when (they) aren't happy.
Sometimes the problem is you are a "mixed pair". One person is a "natural romantic". The kind that remembers details about first dates, anniversaries, and always looking for ways to keep the romance alive. The other person is considers them self to be a realistic "practical person". They feel it's perfectly natural to expect romance and passion to fizzle. In fact in their view being in a committed relationship means; You get to RELAX and stop behaving like teenagers in love without the fear of your mate leaving you. Very much like a professor becoming "tenured".
Ideally it would great if two romantics got together or two practical people got together. Both would have the same expectations!
The reason this rarely happens is because "practical people" behave like "romantic people" at the start of all new relationships!
By the time the romantic discovers this they are "emotionally invested" in the relationship.
Sure one person could start leaving love notes on the bathroom mirrors, offering massages, suggesting date nights and weekend getaways, occasionally taking bubble baths or showers together, spontaneously initiating sexual acts in other rooms of the house...etc
However if there's no reciprocation of efforts eventually they quit.
There are only two ways to experience joy and peace of mind in relationships: We either get what we want or learn to be happy with what we have. Accept them as (is) or move on.