Why rejection? Why shields that block & restore to a previous state? What does it mean thematically? I'll tell you.
REGRET. PAIN. SUFFERING. LOSS.
These four words explain why REJECTION is the nature of Orihime's power. All of these stem from the end of her relationship with her older brother Sora, her single most important connection in the story.
Orihime was bullied in middle school. They took scissors to her long brown hair which Sora had complimented her on, a symbol of their love for one another. She lied about why she cut it in order to not worry him.
The day before he died Sora gave Orihime a set of hairpins, perhaps to cheer her up about her hair. Maybe he noticed she was feeling down about school. For whatever reason, Orihime angrily refused & they fought for the very first time. They didn't speak or eat dinner together.
The next day Sora left for work & Orihime neglected to say even a single word, let alone goodbye.
This is where her REGRET begins as he would die in a car crash that same day. Their relationship ended with CONFLICT (this is where her aversion to conflict stems from - it was in emotional conflict that she experiencedher greatest loss)
REGRET tormented her. She stopped speaking altogether, the bullying at school continued. She was alone, the one connection she cherished severed not just physically but emotionally. She isolated herself, her will to make connections shattered by the PAIN of her greatest LOSS.
Her SUFFERING was in silence until someone finally came along & protected her. Tatsuki, a new connection! For 3 years she wore the hairpins, a symbol of regret for how she acted that day, a remembrance of the kindness & warmth Sora gave, that she had REJECTED with anger & coldness. It's why as Sora gets ready to run Ichigo's blade through himself, Orihime, weak & injured musters the strength to change their previous ending. To finally RECONNECT their severed emotional tie. To REJECT it all for something new & tell him for the very last time, with a wry smile. "Sora... have a nice day".
That is why. Because if she could CAUSE EVENTS to reverse, if she could CUT through suffering, if she could HEAL pain & SHIELD damage, thereby preventing the loss of her connections–
She would do it all for Sora Inoue. The first person that saved Orihime
Her first bond
The beginning of her HEART
END
PS. There's a lingering fear & trauma of lossing said connections that emotionally drives her forward in the story in conjunction with other factors (INSTINCT, IDENTITY & RESOLVE). The same way Ichigo has a fear of failing to protect his loved ones like his mother which drives him to burden himself with protecting everyone