It had started when she'd stood up to Hawkmoth on that faithful day, and his heart bloomed harder in love than he'd ever have expected it to. He didn't really see her much outside of his suit, but whenever, he did, he felt like he was as reduced to awkward shyness as he could possibly be and unsure of how much he could say half the time.
Adrien saw more than spots whenever he looked at her, saw more than just the suit, more than just her strength and heroism; he saw someone who could be just as shy and awkward as he was though he could scarcely believe it on the day that they'd first met as just Adrien and Ladybug.
He saw a woman who could be just as vulnerable and human as she could be brave and strong; Adrien saw a ray of hope that latched itself somewhere on her, that gave him the confidence that they'd all get through this, and one day, Hawkmoth won't be a threat to all of Paris.
Adrien could barely believe how she'd somehow managed to steal his heart with just a sign of being brave, of enduring her own insecurities to push past everything holding her back, and yet his heart still fell for her all over again every single time that he saw her. He could barely believe how a crush could become something quite like this.
Even as his crush, she was brave even during awkward moments, and she was his inspiration to keep trying somehow. She was all that a superhero should be, practically real to the touch with her honest heart, and just strong enough to be brave past everything tearing her down.
Adrien loved every second with her, counted his lucky stars that she'd even been near enough to hold him since being in her arms, was amazing, the few times that it had happened. He could barely believe how she'd caught him that day metaphorically. Every touch was like static, every nerve ending practically on fire and inflamed, whenever she was near. She was more than just a crush in his eyes, she was a friend, someone to look up to, and yet at the end of the day, still believe in so wholeheartedly.
Ladybug was a hidden pool of strength to anyone that needed it since not only was she strong, but she was real; being honest and genuine is much more inspiring than any facades and while Ladybug didn't wear all of her heart on her sleeve since she had to tuck some of it away in the corners to keep her identity protected, he loved all that he could see of it. May be that was why she was his crush, the one person that stole his heart and kept on taking it for joyrides on her yoyo, and the one that he was thankful to call his best friend in his suit and somehow even a hint of a friend outside of it. Adrien knew that even if his feelings weren't requited, even if her heart didn't sing in tempo with his, that he was glad to call her a friend and just may be a crush too.