In-canon, both Ladybug and Chat Noir are (not-so) blissfully unaware of their secret identities, thinking that it would keep them from become too attached to one another.
Marinette has a crush on Chat Noir's secret identity Adrien, but it far too afraid of rejection to tell him how she feels. Chat Noir has feelings for Ladybug, broadcasting them through casual flirting, but Ladybug's interest in Adrien and a barrier of professionalism keeping them apart.
Chat Noir has encountered Marinette outside of her secret identity more than once, such as in "Evillustrator" when Ladybug asks her to look after her secret identity, since the villain of the episode Evillustrator has his attention on her.
Because they do not know each other's secret identities, this removes those barriers when Marinette (as Marinette) talks to Chat Noir (as Chat Noir). Marinette is able to talk to Chat at her own level, allowing her to talk openly and confidently to Chat as opposed to her usual nervous demeanor around Adrien. This could even cause her to admit her feelings for Adrien to him, allowing her to essentially confess her feelings to him without even knowing it.
Chat Noir is able to speak to Marinette in his usually manner without the need to keep his image, and would even be able to confess his more sincere feelings to Ladybug without even knowing it.
With this openness to one another, this could cause the two to eventually fall in love under these identities, eventually leading to the realization that those they longed for where also the ones they then fell in love with.