Chat Noir makes it a habit to tell Ladybug he loves her every time he sees her and every time they part ways. This is because he understands the fragility of life considering what had happened to his mother - someone who is so so very precious to him - and knowing that he would never see her again broke him.
For a long time, there was a big hole inside his heart but it had slowly mended itself ever since he met Ladybug and got to know her better. Ladybug has become someone very precious to him and he does not want to lose her. He knows perfectly well what their roles as superheroes in Paris entail and that they are constantly in danger of losing their lives. That is why he would jump on any opportunity that comes his way to tell her that he loves her.
Ladybug was very surprised the first time he said the three words and had quickly shot him down because she took it as a confession. Surprisingly, Chat Noir was not fazed by the rejection and had instead, given her a huge grin and bid his farewell before leaping off towards the direction of his home. Ladybug was left confused as she stared at his retreating back.
It did not take long for Ladybug to realize that there was a deeper meaning to those three words and how much they meant to Chat Noir when he repeatedly tells her he loves her. She did not dare ask what happened nor did she want to take a wild guess, so whenever Chat Noir does his usual greetings and farewells, she would always repeat the same response:
"I know, kitty, I know. I love you too."