When he first met ladybug, he wasn't wowed. That's not to say that he thought badly of her, but that he saw her as shy and needing reassurance.
Ladybug might have been scared, but she didn't let it stop her. Unlike him, she could face her fears and stand tall. She was smart, really good at figuring out an akumas weakness and coming up with plans on the fly. Adrien (or rather Chat Noir) fell in love with her for all these qualities.
Loving Ladybug was as easy as breathing. She just fit, like she'd always been a part of his life. They weren't allowed to know some parts of their lives, but he'd treasure what they could share
Later, he knows she's passionate about injustice, likes the color pink and is somewhat of a control freak. The last part kind of unsettled him. He'd had enough of it from his father. But he tried to push it off his mind, Because he trusted ladybug.
Maybe he was wrong to, because when they lost master fu, she changed, just like his father did when mom died. And then he was walking on eggshells and kept out of the loop. His life as chat noir mirrored his civilian life more and more. It wasn't an escape for him anymore.
And then Ladybug lost all the other miraculous, and he had to pick up the pieces. He couldn't help but feel grateful that she was finally relying on him, and then chastised himself for thinking that. Ladybug was vulnerable now, he had to be the bigger person
Things didn't return to the way they were, and thinking on it, it's for the better.