the fate route date, subsequent argument with saber, and confrontation with gilgamesh form the biggest turning point in shirou & saber’s relationship and especially how shirou views saber. up until that point even though he’s taken great issue with saber treating herself as a king(an ideal) instead of a girl(a person), he tries to get her to stop doing that by forcing his concept of how to be a person on her, which is a very poorly formed concept because he spent the past 10 years trying to be an ideal rather than a person too. while he genuinely tried to help her all this time, it was very much based on a fumbling “normal people(girls) should do this right???” without actually understanding or trying to understand saber and how she can be helped, reflecting fate route’s overarching question of “what does it truly mean to be saved?” and its answer that there is no cookie cutter answer that works for everyone.
but during the date it becomes clearer than ever that both of them have no fucking clue how to be normal average people instead of the ideals they live by, so while they cautiously enjoyed themselves the date felt uncomfortable and forced to both of them and that tension exploded on the bridge into what was basically a back and forth of “no YOU take better care of yourself”. when shirou later that night returns to the bridge it is with the resolve to stop forcing what he thinks saber should do as a person on her and instead stand by her while they both figure out how to live as their own people. it’s this moment where shirou becomes fully determined to not just convince saber to live as a person instead of an ideal, but to also understand her as a person rather than a representation of and outlet for his own heroic ideal. the switch is from “I want to save saber because I want(need) to save someone” to “I want to save saber because I love saber”.
at that moment gilgamesh appears as a test to that resolve. gilgamesh here is pretty much shirou’s exact opposite: immense power, infinite swords (as opposed to the only one shirou is able to make this route), and a complete disregard for saber’s autonomy. while shirou has also disregarded saber’s wishes and forced his own idea of how she should act on her before, this is where he shows that a) it was out of good will and b) he realises it was wrong and will learn from it and truly listen to her from now on. this carries over into the aftermath where (in realta nua) once they’re home saber asks if they can sleep in the same bed that night. it’s a degree of vulnerability that saber hasn’t willingly shown shirou before and the accompanying trust that shirou will understand & accept it, because he’s now shown undeniably that he loves & cares about her and not what she represents to or can do for him.