It is human-nature to seek closeness with other human beings - for each individual to want a 'special someone' just for them. Some people do this to the exclusion of all others, their codependency becoming a tight-knit unit of two with no room for self or for others. The lack of air between this couple often suffocates individualism - and yet seems to provide a mutually beneficial comfort for both partners. Some couples are happy with this arrangement.
Other couples seem to have no commonality in their lives. The coming together is infrequent and at best polite. They are ships passing. Again, this might suit both partners and might be exactly how they want their relationship to be.
Other individuals choose a partner with whom they have things in common so that they can have shared interests and activities and can pursue these together whilst also doing things apart. In this type of coupling there will be room for individuality, room for separateness and room for personal growth. And yet even in this balanced, equitable coupling there may be a fear of abandonment and a fear of engulfment working in opposition to each other. The fear of abandonment in the relationship maintains the longing for closeness; the fear of engulfment prevents the loss of self. The separateness and autonomy are there to preserve the individual in the relationship and paradoxically to allow closeness to exist. If the togetherness is too close, there is no distance to travel to find each other.
Esther Perel in her excellent book, Mating in Captivity states 'with too much distance, there can be no connection. But too much merging eradicates the separateness of two distinct individuals. Then there is nothing more to transcend, no bridge to walk on, no one to visit on the other side. When people become fused - when two become one - connection can no longer happen. There is no one to connect with. Thus separateness is a precondition for connection: this is the essential paradox of intimacy and sex.'
The fit of a couple is as unique as the individuals themselves. There is no right or wrong in how a couple fits together so long as it works for both of them. When one or other partner is no longer happy or when disharmony is prevalent, perhaps it is time to seek help from a couple counsellor who can facilitate the communication needed between you to move forward more harmoniously.