I believe we fall in love based upon (who the person is) and what they represent. In other words if they weren't who they were we probably would not have fallen for them. By the same token we are constantly growing and evolving which means our needs or desires change over time. The perfect mate for you at age 21 may not be the type of person you want to be with at 41. Change is the one certainty in life.
Ultimately love remains when both people continue to want the same things at the same pace. We're either growing together or growing apart. Communication helps to determine whether or not a couple wants the same things or have the same priorities. The rest of it comes down to making the effort to keep romance and passion alive. Again it can't be done by one person alone. Both people have to want it.
Too many of us follow the practice of treating "the new" better than "the tried & true". We bend over backwards to "impress" each other in the beginning of a new relationship. After there is an emotional investment or commitment we reveal our "authentic self". It's not uncommon to hear someone say, "He or She is not the same person I fell in love with". I wrote about this backwards approach to love awhile back.
"It's easier to maintain a fire than it is to reignite a spark!"