Based on the typical relationship advise: "Don't try to change your partner" and such, I would say that many people want to see their romantic partners behaving just a little bit differently. It is a common thinking pattern.
For example, a person may be wanting for the strong and independent partner to be more caring and compassionate. Or wanting for a partner to spend more time listening, or being more punctual, etc.
These thoughts can be summarized as "Our relationship would be perfect if only he/she did XYZ". In other words, people expects to experience relationship bliss when their partner complies with their wishes.
It seems to me that the XYZ is always a moving target, and can never actually be fulfilled, a sort of mirage, just out of reach. This makes me think these thoughts could be caused by some neurochemical process.
Is expectation of relationship bliss, conditional on XYZ is just a side effect of some neurochemical process? Is it an expression of seeking novelty in a relationship ("same, but different")?
Edit: To clarify what I mean by Neurochemical: If a person ingests 8 shots of hard liquor, there's a new chemical -alcohol introduced into blood circulation. In a short time alcohol will affect cognition regardless of any protests from the person's higher order cognition. A person cannot will themselves to not feel the effects of the chemical and act normal.
The ingested chemical affects the brain (neurochemical?). Until the chemical is cleared from the circulation or neutralized, the person will continue experiencing cognitive effects of the chemical compound.
By neurochemical process I meant an indigenous process by which brain can release a brain affecting chemical/hormone that will act according to the model above. Adrenaline in fight/flight response is an example of such chemical. Maybe Oxytocin is another example. I know of words like psychoactive, psychedelic, neuromodulator and less scientific terms like "mind altering", but none of these seemed to fit.