The sweetest sound was never produced by pure bliss, untainted, unbroken, untransformed by the woes of the world. Bliss can only stagnate a situation, not push it beyond itself, and her sound, her heart song, still captures his heart and mind, and all that he can see and hear is her. Marinette sounds like no other, not like the bliss that so many cling to their age, and not at all, the tortured sound of one broken beyond relief.
She's like a phoenix in Luka's eyes. She's come alive to the changing of pain and sorrow to strength, to the realizations that trauma forces on you, and she's powerful amid it all. Marinette has went through so much, the beauty of her heart song and the ache of its tremors attests to such. Luka just can't place what beat down the walls and transformed her into the compassionate, the empathetic woman that she's been since he first met her, when she stammered through getting his attention.
He's drawn like a moth to a flame as her beauty is a beauty held in strength and in wisdom; wisdom can not be taught in schools though you can study the wise. Wisdom is taught through hardship and struggle and just living. Marinette's already wise, not as wise as those much, much older than her, but certainly wiser than anyone her age normally is. Luka's drawn to that in her, despite knowing that with that wisdom, with that beauty to her heart song, that she's had to have went through so much.
He can't know that her pain as been dealing with being a superhero, the initial insecurity, the forced responsibility, and the ache of when things don't go right or even the times when she's had to deal with heartbreak without answers to guide her. It's easy to have trust that maybe something will change, but it's hard to have absolute faith in a certain change of events that you can only have will, especially when you are tempted by just how easy it would first seem if things went another way.
Luka doesn't breathe a word of his questions, not with her before him, and aching from something not quite like being a superhero, but pretty painful anyway. He doesn't ask when she just listens and takes comfort in her heart song, not knowing that the struggle, the pain, and the heartbreak are exactly what's made it so beautiful. Already, it feels as if every time that he sees her, she's grown another layer closer to being just a bit more wise and yet a bit more stung by reality as well.
He doesn't dare break that glass that she holds up as a protection for the questions that others are always tempted to ask. Maturity is bred by knowledge and often times wisdom, and so he knows that as she continues to grow up so fast that something must be going on beneath the surface, but yet he only knows this to the extent that he can feel it. Words don't come quite as easy as strumming the right notes to convey a heart song, even when hurting and aching, and Luka would never consider himself a master poet or a skilled songwriter or someone who just knows exactly how to say what he feels and notices. Words can fail him more often than he'd be able to explain, but something's there in her heart song regardless.
Luka's spellbound by the sound, by just being near, and even though her heart aches and throbs, he finds the beauty in it and the unimaginable strength already sturdy and growing within her with every beat of her heart. He couldn't even pretend that his feelings aren't strong or that they won't last for a while as he just listens and listens and listens all over again. It's beautiful in a way that words from his lips could never convey.