Luigi lacked confidence. It was a fact that didn't really have to be stated aloud or put into tangible words; one only needed to stay around him for long enough to notice his stuttering voice, his awkward movements, how he rarely looked people straight on. Yes, he had been getting better at it, but it still stood that he just wasn't the most confident person. He really wanted to be, though, or to at least have enough confidence to look people in the eye and speak without falter.
But that wasn't the case, and with where Luigi stood, he didn't stand a chance to even speak with Rosalina. No, it wasn't because she was rude or arrogant or the kind to bring others down––not in the slightest; rather Luigi would always find himself too inadequate in her presence. He couldn't help but look at her and see everything he wanted to be: she was polite and yet unafraid to voice her opinion; she was soft-spoken and yet each of her words carried weight, directing others to hear her and respect her; her very aura spoke of a steely strength, able to endure and face more than Luigi ever could; and her gaze alone showed an inner independence, a confidence that could never be brought down. In the end, Rosalina and Luigi were of similar disposition––introverted, familial, caring––but Rosalina had an edge to her. She was the tempered version of Luigi, honed to the idealized form.
It shook him deeply when he first met Rosalina, to see what he could be, sending him to grasp for some way to affirm himself: mainly searching for Power Stars, but that ended in a bust when Luigi couldn't even scrounge up the courage to see Rosalina again and had to delegate the process to Mario, making the promise of telling her that Luigi had been the one to get them. The whole thing reeked of desperation, screamed of a piteous plea to "Please look at me; I swear I'm capable!" and yet shrinking to a quivering, small mess when reality came knocking.
He supposed that he just really wanted validation from Rosalina: a sign that he was, on some level, par with her––that he could stand without falling to his knees, unable to bear the weight of life without help, that he was somebody.
But, he also supposed that, now that Rosalina was joining their group's adventures, he wasn't her equal at all; she was making a name out of herself, earning admiration for whom she was, while Luigi was left to catch up like he always did. But this time, rather than seemingly making progress, he simply got further and further behind, until eventually all he could do was look at her and think "What if?" What if he could make big strives rather than having his own self hold him back? What if he could stand up rather than cower in a corner? What if he could face himself?
The answers didn't even matter, however; Luigi was stuck, trapped underneath yet another shadow, and he was too much of a coward to step out of it.
However, at this point, Luigi just accepted it as life.