The only way it happens is if (both people) want it!
More often than not that isn't the case. One person has a low libido or an overall lack of interest in romance, passion, or sex. The other person eventually gives up on putting in effort that is not being reciprocated. Sometimes people "fall out of love".
Eventually they become roommates with the same last name.
It's easier to maintain a fire than it is to reignite a spark!
As I stated if (both people) want to put the effort into becoming romantically connected there are tons of help available depending on what they're open to. Initially it starts with having more one on one time, snuggling while watching TV, holding hands while riding in the car or taking a walk. Taking bubble baths or showers together, cooking meals together, having date nights, getaways/vacations, and so forth. There are also adult stores and therapy available too.
However if only one person believes there is a "problem" they may have to decide whether or not it's a "deal breaker" if things don't change. People only "change" when (they) are unhappy.
There are only two ways to experience joy and peace of mind in relationships: We either get what we want or we learn to be happy with what we have. Accept them as (is) or move on.
The choice is up to us!
Hopefully (both people) want to keep the magic alive!