Alain de Botton explains why we are cruelest to the ones we are closest to. Most of it has to do with the fact that we have such devastatingly high expectations for them to meet our devastatingly deep neediness. A section on “Pessimism” from The School of Life’s book, Relationships

No one can disappoint and upset us as much as the person we’re in a relationship with–for of no one do we have higher hopes. It’s because we are so dangerously optimistic that we call them a c***, a s***head, or a weakling. The intensity of the disappointment and frustration is dependent on the prior massive investment of hope. It’s one of the odder gifts of love. 

A solution to our distress and agitation lies in a curious area: with a philosophy of pessimism. It’s an odd and unappealing thought. Pessimism sounds very unattractive. It’s associated with failure; it’s usually what gets in the way of better things. But when it comes to relationships, expectations are the enemies of love.

…Love is the discovery of harmony in some very specific areas–but to continue with this expectation is to doom hope to a slow death. Every relationship will necessarily involve the discovery of a huge number of areas of divergence. It will feel as if you are growing apart and that the precious unity you knew during the weekend in Paris is being destroyed. But what is happening should really be seen under a much less alarming description: disagreement is what happens when love succeeds and you get to know someone close up across the full range of their life…

Being disappointing is not a specific failure of individuals: it is a universal phenomenon. A partner has their unique and specific problems, but anyone else will have their own different, equally maddening, repertoire of hang-ups, failings, and obsessions. What is genuinely special about our partners is that we have come to know their worst sides so well. The charm of a new person rests on the fact that we don’t yet know them well enough to understand how they too could drive us mad.