"How can you love someone entirely if you don't know them entirely?" - Dylan, Lovesick.
A lot of people have this idea the Love is simply a feeling, like sadness or joy. While there are sensations involved with love and there are chemicals that make feel attached to someone, Love still remains a choice.
I've been thinking a lot about happiness and love and how many people have told me they go hand in hand. Media and society have made these two things almost synonymous, and they are big feelings we can sustain indefinitely and that they are SO easy when you find The One. The more I think about it and compare the things I have been told, the more I realize this is just one more unrealistic piece of information we have consumed from today's society. All these "Life Coaches" are spouting about how easy happiness and relationships are once you buy their trainings and follow their "special skills" you will be happy and find "The One". I can not bring myself to believe any of the things they are preaching. There is no magic skill that makes love easy. There is no "The One". There are too many human variables for love to be easy.
The most accurate theory of love I have found is Sternberg's Triangular theory. I deem it as accurate because he identifies the main components of love and how having these components or not creates different types of love. These components address the cross-cultural differences in how love is viewed and maintained. There is an empty love that is just being committed to someone without intimacy or passion (similar to the experience of an arranged marriage). It also discusses how a shift in these components leads to a shift in the type of love (shifting from friendship love to companionate love). This is what is missing from a lot of the ideas we are fed. That love changes and shifts with the people who are experiencing. That consummate love (the ideal relationship made up of all three components: Intimacy, Passion, Commitment) is possible and maybe easy to achieve with a compatible person. However, Sternberg believed that "Love is a verb and consummate love is harder to maintain, as all the components of love must be put into action." We must practice and work on our commitment, passion, and intimacy.
In all my research my favorite answer to how do you stay in love after all this time has been that he has loved every version of her. Each decade she grew into a new woman and he chose to love and grow with her. The only way to keep feeling the sparks and feel good feelings is to pursue experiences together and grow together. To do this you have to make the choice that this is your person and you are going to commit to them.
This is not to say that one day you won't naturally grow apart and become different people. There is no way to predict who you will become. You will never know someone entirely because you can not learn every detail of their past (if you did learn, you can't change or influence it) and you will never be able to predict who they will become. However, you can entirely love them in their entirety by choosing them every day, by practicing passion and intimacy, and investing in them.
More detailed explanation of Sternburg's Triangular Theory of Love:
https://www.verywellmind.com/types-of-love-we-experience-2303200