You are the youngest child of a very important family in fantasy feudal Europe, quite close to the throne, and trusted enough that you were your monarch's sole childhood playmate. The most important characteristic of nobility in your society is the ability of nobles to perform magic. Your mother is a living legend, a pinnacle of success as a noble, a soldier, a woman, and a mage. Your elder sisters are likewise both beautiful and in the case of one, highly successful as a mage.
Your favorite sister is the only outwardly affectionate member of your direct family, and is deathly ill, incapable of doing much of anything. Your other sister is harsh, cynical, and apparently delights in degrading you. Your mother has raised you from a young age as if you were a soldier, burdening you with seemingly insurmountable expectations, which have only been worsened by your complete failure to progress as a mage or develop as a woman, to the point where you were promised off in a political marriage to a man twice your age because there was no apparent hope of you becoming noteworthy in your own right.
You managed to get sent off to mage school despite an apparent complete lack of aptitude. While there, essentially all of your peers either ignore you or deride you. No boy your age shows any interest in you. Your teachers are at best indifferent, and you continue to make no progress. All of your attempts to perform magic -- aka, to demonstrate your worth to society -- are apparent failures. Worse, a scion of one of your family's oldest rivals is also attending, and makes a point of continually tearing you down on every possible front, while simultaneously being notably successful in each.
Nevertheless, you persevere until the biggest point in your career as a budding mage, the summoning of your familiar, which is supposed to prove definitively that you are magical, and which also serves to foreshadow your power. Your peers all succeed without incident, and some -- including your rival -- enjoy remarkable success. At first you appear to have failed, but then (as far as you can tell) scrape past with a success in name only. Your pre-existing self-worth issues lead you to follow the assumptions of your peers in treating a human familiar as a failure rather than something notable (because, after all, humans are apex predators and highly adaptable -- not that anyone considers it in those terms).
But then, a bright spot. The familiar you summoned is a human boy, and better yet, he acknowledges you as attractive! Granted, it's been so long that you don't really know how to deal with any sort of positive treatment, even when it's basically the off-handed complement of a hormonal, pervy teenaged boy being attracted to you. But then the nitwit not only falls into line with your peers in insulting and degrading you, but he continually vacillates in his attraction to you, to the point of flipflopping from professing love one moment to admiring a third-party example of the most common superpower the next.
In short, you've begun crushing on perhaps the only person to give you any real positive attention (and the 'positive' there is questionable, to boot), and the bastard not only continually participates in challenging your already incredibly tenuous place in the social order and your worth as a person, but also pushes more basic buttons by alternating between returning your affections and ignoring you for the sake of other girls because of his incredibly monodimensional matrix of physical attractiveness.
You respond to disobedience, failure, and disrespect in the way which your family and society have taught you is appropriate.
That aside, I can almost read the LN treatment of her abuse of Saito as being commentary on the society (most of the reason it's apparently treated as humorous or lighthearted in any of the incarnations is because the
treat it that way), but I'm reluctant to ascribe that much creative depth to any author who makes more than a third of their recurring characters underaged girls with chests like that.