Listening to heart songs did not always mean that some songs didn't mesh or fall together in a perfect line, a perfect order. Sometimes it was easier to tell by listening something much greater, though he wasn't entirely sure if it was a mere coincidence or if they just happened to have similar ones or if somehow the girl that stole his heart with her awkward and fumbled nerves and a sweet, endearing heart that held an absolutely gorgeous song to its beat was the same as the superhero that ran through Paris saving the day.
Ladybug rarely was close by Luka's side, and thus, he'd rarely got a chance to listen to her heart song or see just what emotions fluttered through her face. She was confident where she was once scared, brave where she once was paralyzed in unsurety, and that's what made Ladybug so endearing, what caught people as if spell bound by her, though not all did see that her bravery had an element pushed on and strengthened by sheer force of will.
Ladybug was just an ordinary girl beneath that mask, though whether Luka knew the identity or not hiding within, he couldn't say. It was impossible to get more than just an idea of what the mask conceals without asking her, asking if she knew him, or at least knew him a little better than a normal superhero would, so he watched and tried to ignore the occasional fluttering of his heart.
When you strike out on love, your mind may wander with other possibilities, but it doesn't get far. Something holds you back from considering irrational ways of making you feel just a little more loved and appreciated. Luka strummed a tune that reminded him of Ladybug's heart song, of just that one time that he was close to her. It kept him going as he tried to ignore how much he wished Marinette were beside him with a smile that he longed to bring to her lips.
You can't just love and forget that you loved, even when the feelings were onesided anyway. He has to fight the burning curiosity of how well they might have been together, so he turns his eyes and attention on Ladybug and just hopes that his heart will stop fluttering after the impossible. Marinette likes Adrien, clearly, and Ladybug is a superhero. Superheroes don't date civilians unless this is another retelling of comic books when that change sometimes slips in.
Luka doesn't fit the mold and doesn't think Ladybug does either. She's too careful in her decisions no matter how quick she has to make them during a fight. Ladybug considers more, probably rethinks and overthinks more too, when she's not in the middle of a fight. He knows that she'd know better than risk endangering someone she cares about, but still he feels that burning, desperate flicker that just wants to stop longing for Marinette to like him back, and hope to be with Ladybug.