A relationship that stands the test of time goes through all the motions - worry, occasional loss of interest from one or both partners, uncertainty, lovers fights, etc.
What keeps partners together through all of this is their ability to stick together and work through it all, using their own personal formula, custom-built for just that marriage/relationship.
Of course, this is because every marriage and relationship, no matter the similarity it bears with others, is a separate, unique entity that should ideally be run on the terms of the two people involved in it.
So coursing through happy and sad times in that relationship, or how things work in it, rests solely on the pair in it, and the good influences they may occasionally or regularly permit.
Regardless of the uniqueness of all individual relationships, there are behavioural patterns that have been found to negatively impact the balance of relationships, and there are also stuff that many couples do, that have been found to generate more happiness in their relationship.
Uwanma Odefa on her vlog,"Love, Life and Everything In Between" points out 5 of these things in the video below: