Canonically, both Ladybug and Chat Noir are unaware of each other's secret identities, thinking that it would keep them from putting their friends and families in danger.
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, not knowing her identity is Marinette, broadcasting them through casual flirting, but Ladybug's interest in Adrien and a barrier of professionalism keeping them apart.
Ladybug and Adrien (as Ladybug and Adrien) have encountered each other on more than one occasion, usually with Ladybug "saving" an Adrien in danger in order to protect him.
Because they do not know each other's secret identities, this removes those barriers when Ladybug (as Ladybug) talks to Adrien (as Adrien). Ladybug is able to talk to Adrien without clamming up like usual, her protected identity keeping her confident and safe from the idea of rejection and embarrassment from Adrien, however she does get a bit nervous at times even as Ladybug. Adrien is able to start over with Ladybug, being able to talk to Ladybug in a less casual manner without the barrier of professionalism.
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 loved where also closer to them than they realized.