Expectations In Context

I’m curious to better understand the idea that it’s unreasonable and ineffective to have expectations (i.e. a manual) for other people. Specifically, I’m interested in clarifying the difference between employer and employee, or a manufacturer/vendor and buyer (where typically, expectations are clearly delineated and agreed upon contractually, and both parties are expected to fulfill their agreement), and committed romantic partners – especially married couples – who also enter into a contractual agreement (seen most plainly in divorce proceedings). Why not make explicit the expectations we have for our partners? Why not delineate which responsibilities are theirs to fulfill, and which are ours? Relationships, especially when there are shared resources, are in many ways more similar to business partnerships than romantic dalliances- it seems more effective to communicate expectations up front than to enter into a binding agreement with someone by merely deciding to take on whatever consequences arise, come what may. What am I missing????