There's a scene in the movie Don Jon where a few months into their relationship Scarlett Johansson and Joseph Gordon Levitt, playing two lovers from Jersey, are shopping at Target and Levitt's character Jon says he needs to go grab some Swiff-a pads. Johansson's character Barbara, completely incredulous, suggests that Jon can hire someone to do that for him, baby. Jon says he enjoys cleaning, and Barbara reacts disgustedly, saying the discussion is over--"We're not talking about this anymore"--leading the audience to believe either Barbara feels Jon is too good to clean his own house or more likely that she is too good to be dating someone who cleans his own house.
The premise of this movie is slightly ridiculous, as are the characters, but when watching this scene I thought about how much the core conflict of situation--i.e. Johansson wants Levitt to be someone he is not, and cannot deal healthily with the reality that he isn't an idea, a fantasy, but an actual human being--happens in likely all relationships. In relationships, there comes a time when the honeymoon phase, as they call it, wears off and stressful situations arise and our partners react in ways we aren't used to. In response, we feel upset, sometimes even betrayed, like we don't even know this person.
I can imagine two reasons this is upsetting. First, the situation, or our partner's reaction, makes us feel something that isn't comfortable for us to feel--like that the relationship isn't stable, maybe the person isn't as committed to us as we thought, or that we're with someone who is capable of x and that is something we had previously labeled a "deal breaker." Additionally, maybe it makes us sense something about ourselves that we cannot deal with--I'm not as smart, capable, good...as I thought I am. This threatens my sense of identity, self.
Second, we want to maintain our illusion of who our partner is because if we had to add this new thing we've seen into the equation we may not actually be able to love and accept him or her completely. This schism causes frustration. (Note: Both one and two are things likely happening subconsciously, at least at first.)
When we first meet people, we have no choice but to fill in the blanks of all the things we don't know about them with many unconscious and some conscious assumptions. This is called making a heuristic, and humans do this unconsciously and constantly. An example would be that maybe we've never seen our partner handle an infant, but we've seen him or her play with a puppy so we unconsciously assume that it'd be the same with kids. Only when we see him or her play with a child can we replace the heuristic.
These assumptions, however, create a problem when we make them concrete and we decide our partner fully is what he or she seems to be during the first few months. These months are mostly good and exciting and stimulate the best version of our new partner to come out. The sample size is too small to really know who all the parts of this new person are, as, and this is important, a person is the entire spectrum of everything they're capable of thinking, feeling, and doing during all the stimuli the world can throw at them. Because of this fact, it takes a really, really long time to know someone.
If we hold the first few months or even years as a frozen sculpture of who our partner is, we will often find ourselves in a break up because invariably and inevitably we'll see all of the person over time and won't like those sides as much, as they will come from more negative stressful stimuli. We'll start to think or say things like "This isn't like you" or "He's not like that" when the reality is we're still getting to know the person and we don't know what he or she is like; we know only who we want him or her to be.