There is a particular longing that has no object. You see it sometimes in people who have everything they said they wanted — the relationship, the apartment, the decade of accumulated belonging — and who feel, nonetheless, a persistent, low-grade sense of something missing. They cannot name it. They feel guilty for it. It does not name itself.
This kind of longing is not about the partner. It is, more precisely, about the gap between what one imagined relationship would feel like and what it actually feels like — and this gap is, to varying degrees, universal. No actual relationship can fully satisfy the imaginary relationship that preceded it. The imaginary relationship is always the more ardent one: uncompromised by tiredness, by repetition, by the particular way another person leaves their shoes in the corridor or cannot quite hear you the first time you speak or has opinions about things you don't care about and no opinions about things you do.
The French speak of le manque — the lack, the gap — as fundamental to desire. The psychoanalytic tradition, from Freud to Lacan to contemporary relational thinkers, has generally agreed that we want most persistently what we cannot fully have or fully hold. This is not a counsel for despair. It is, perhaps, a counsel for honesty. To long within a relationship — for more closeness, for an earlier version of the other person, for an intensity that life has muted — is not necessarily evidence of a failed relationship. It may be evidence of a living one.