No kid likes his toys, always his friend's toys. No teen likes his college, always his friend's college. No adult likes their marriage, always their neighbour's marriage.
You have a tendency to overestimate the value of something that you do not have and underestimate the value of what you do have. Because when you do have something — the flaws are clear. You know the thing inside out, you know where it lacks, where it frustrates and disappoints you. You know that your professors are bastards and your spouse doesn't shower after taking a dump. You don't know that about others' professors and spouses tho. You don't have the privilege to understand what you don't have. So inevitably, you start to treat it as the solution to all the flaws of what you do have.
The psychology behind this is "hedonic adaptation". No matter how noble something is, over time as you keep getting this same stimulus, it becomes less and less exciting. Once something becomes routine, it becomes invisible. So you crave new stimuli, something unexpected, something that is not routine. Just like you don't notice the sound of fans, cafe noises, rain and so on —They become routine, and so — invisible, so do also the pleasure of repeating stimuli.
Take the case for jokes. The first time you hear a good joke, it makes your stomach ache. Second time, it makes you laugh a lot. Third time, a bit less. And eventually you stop laughing at all. Similarly, You will see people buy expensive things and eventually they start to treat them like their cheap counterparts. Something similar happens in economics that we call diminishing marginal utility, where each successive unit of an item gives you lesser satisfaction than the unit before. [1]
When someone gives you a present for the first time, it's special. But if they keep giving you the same present, you lose interest. But you CAN bypass someone's adaptation (to an extent) by just giving them irregular and unexpected presents. Keeps them craving, like gambling.
But the problem is with sensitive things. Trouble starts when you adapt to your significant other and start craving for a new stimulus by projecting an illusion of perfection on someone else. You don't really hate your significant other, you just want all their flaws gone, plus their good traits remain. They can't do it, so maybe someone else can. Right?
This is a lethal delusion if you're not aware of it. It keeps you in a limbo questioning the validity of your emotions, when it's just "the grass is greener on the other side" bias in play. Trouble is when you start to take your urges to escape seriously and turn it into urges to be unfaithful. [2]
The first step in omitting it is to be aware of it.